Encryption-based cloud storage adds work that an ordinary file copy does not have. Before a single byte leaves your device, the file is encrypted; during transfer it may be chunked and uploaded in parts; afterwards the client compares its local index with the server copy and reconciles the difference. An upload that appears frozen can be doing any of those steps perfectly. This guide walks through measuring which step is actually stalling before you change anything.
The word "sync" covers two very different directions that get confused constantly. Uploading sends a new local file to the server. Downloading pulls a server-side change down to a device. Reconciling compares the two indexes and resolves the difference, which is what you see when a folder appears on your phone seconds after you renamed it on your laptop. Before you diagnose anything, decide which of the three you are actually waiting on.
Measure before you change anything
Two numbers turn a vague complaint into a usable diagnosis: how long the process has been running, and how much data has moved. If a small file has sat at zero percent for more than a minute, the network path is the problem. If a large file reaches ninety percent and stops, the limit is usually file size or available storage. If a folder shows no progress at all but individual files inside it upload normally, the problem is item count rather than throughput.
Compare one small test file against your real upload. A two-megabyte document that completes in seconds proves your account, your client, and your network path are all functioning, which immediately rules out three of the most suspected causes at once. Test uploads are cheap and far more informative than reading error text.
Check the storage quota before the network
A full storage allowance is the most commonly reported cause of an upload that will not complete, and it is easy to miss because the client often reports the failure as a generic network error. Upload takes space, and version history takes more space than you would expect: every revision of a document occupies storage until it ages out or is cleaned up. A library that was comfortable last month can be effectively full today because nothing was deleted and everything was edited.
Look at the reported usage and the plan you are actually on, not the plan you believe you are on. If usage is close to the limit, remove genuinely obsolete data before you investigate anything technical.
Rule out the network path
Encrypted storage depends on a stable outbound connection, and it degrades more visibly than plain file transfer because it cannot fall back to an unencrypted channel. Corporate proxies, school networks, VPNs, and some guest Wi-Fi setups inspect or block traffic that a normal browser would pass. A captive portal that lets a web page load while blocking other connections produces exactly the symptom people describe as "the app cannot connect".
Test with a different network entirely — a phone hotspot is the fastest reliable control. If the upload completes on the hotspot, the problem is your current network and nothing about your account. If it fails on both, the network is not the cause and you should stop adjusting connection settings. If the transfer only fails right after you complete a Proton Drive login on a new device, a half-finished first sync is usually still running and competing for the same connection.
Understand why large files behave differently
Large files are split into parts, uploaded independently, and reassembled. That design is what allows an interrupted transfer to resume rather than restart, but it also means a file above the current size limit can fail at a stage that looks like a hang. If your largest successes are all well under a certain size and your failures are all above it, you have found a boundary rather than a fault.
Compression helps with documents and archives, and hurts with photographs and video that are already compressed. Adding a zip file to a folder of JPEGs usually makes the transfer slower, not faster.
Watch for mobile background limits
Phone operating systems restrict what an app may do in the background to protect battery. If you start an upload, switch to another app, and the transfer silently stops, this is very often the reason. Allow background activity for the storage client, keep the device on mains power for large transfers, and avoid letting the screen lock mid-upload if you can prevent it.
On mobile data, some clients apply a separate size or time limit to background transfers. If an upload completes on Wi-Fi and stops on mobile data, that policy is the explanation.
Deal with sync conflicts between devices
When two devices are offline at the same time and both receive changes, the next connection has to reconcile them. Depending on the client version and the item involved, this can produce a conflict warning, an item that reverts to an older version, or a sync that pauses and asks you to intervene.
When that happens, do not rapidly alternate between devices trying to force a refresh. Pick one device, bring it online, let it settle, and then check the result on the second device. If two devices genuinely received different edits to the same file, decide which version is correct before saving anything, because the loser of that decision may not be recoverable.
Build a report an engineer can act on
A report that will produce a useful answer includes the client and version, the operating system and its version, the approximate file size that failed, the size of the largest file that succeeded, whether it fails on every network or only one, the reported storage usage against your plan, and the exact wording of any message. "Proton Drive is slow" produces a question; "a 900 MB archive fails on hotel Wi-Fi and succeeds on a hotspot" produces a diagnosis.
Remember what this website can and cannot do. We are an independent fan site, so we cannot inspect your account, raise your storage limit, or reset a sync token on your device. Only Proton's official support channel can act on your account, and everything you prepare here is designed to make that conversation short.
A short order of operations
Confirm which direction you are waiting on. Upload a small test file. Check storage usage against your plan. Try a different network. Compare against the largest file that has ever succeeded. Adjust background activity permissions on mobile. Then write the report. Changing five settings at once, in an order you cannot remember, produces a result nobody can interpret including you.