The symptom is confusing because it contradicts itself: the internet is obviously working, other pages load, messages send, yet one app spins and gives up.
When everything fails, the cause is clear. What's awkward is when almost everything succeeds.
And precisely because only one thing fails, speed is almost never the cause. Different apps take different routes, and one of those routes is broken.
The first check that saves everything
Before touching any setting: try the same app over mobile data, with Wi-Fi switched off.
- Works on mobile, fails on Wi-Fi: the cause is your network. Read on.
- Fails on both: the service is having trouble. Nothing at home needs fixing, and time spent on the router is wasted.
This one minute splits the problem in half, and half of it isn't yours.
Four suspects
If it really is your network, the cause is nearly always one of these four, and each fails distinctively.
1. DNS, the name can't be translated
Apps reach their servers by name, and that name must become a number first. If translation fails for one particular name, only that app goes down.
How it looks: instant failure, no waiting. The message usually mentions a server not being found.
The test: change the device's DNS to a public one and try again, see the guide to changing DNS. How it works is in the guide to DNS, and if DNS itself is struggling, the guide to DNS not responding goes deeper.
2. Blocking
The service is deliberately closed off, by the provider, by a router setting, or by network-level filtering.
How it looks: consistent failure, every time, from every device on that network, but normal on other networks.
How blocking works and how to tell the types apart is in the guide to how site blocking works. If the block is coming from your own house, check the guide to parental controls on a router, these are often on without anyone realising, especially on ISP-supplied equipment.
3. A closed port
Some apps don't use the same door as a browser. Online games, voice calls, security cameras, and work applications often use their own port numbers, and some networks close everything but the common ones.
How it looks: the app opens and menus load, but the main function fails. A game reaches the lobby but can't join; a calling app rings but no audio arrives.
This is most common on office, boarding-house, and public networks. At home, the cause may be CGNAT, see the guide to NAT and CGNAT, and if a port genuinely needs opening, the guide to port forwarding covers it.
4. MTU, packets too large to pass
The least known cause, with the most distinctive symptom.
Data travels in pieces, and there's a size limit for pieces crossing any given path. If the limit set on your equipment exceeds the real limit along the way, large pieces get stuck while small ones pass freely.
How it looks: pages open but images never appear. Apps log in but video won't play. Text arrives, files don't.
The test: lower the MTU in the router's WAN settings, from 1500 to 1492, then to 1400 if needed. This shows up most often on PPPoE connections.
The order to check
Ordered by how quickly each is answered, not by how common it is:
- Try mobile data. Splits the problem in two.
- Try another device on the same Wi-Fi. If only one device is affected, jump to the guide to single-device problems.
- Change DNS and retry. Five minutes, and it closes one possibility entirely.
- Note the shape of the failure. Instant failure points at DNS or blocking; a long wait then giving up points at ports; partly working with missing images points at MTU.
- Turn off VPN and ad filtering, if present. Both frequently block one service while leaving the rest alone, see the guide to VPNs.
- Lower the MTU, if the symptoms match.
Two cases with their own explanation
Everything fails after joining a new Wi-Fi. That isn't a per-app problem, see the guide to Wi-Fi connected but no internet.
An app fails on café or hotel Wi-Fi. Many public networks close most doors except those a browser uses. Voice calls and online games are the usual casualties, and that's deliberate on the operator's part, not a fault. What to watch for on such networks is in the guide to using public Wi-Fi safely.
What won't help
- Restarting the router repeatedly. If once doesn't fix it, ten times won't either. A problem affecting a single app isn't one a restart clears.
- Upgrading your internet plan. Speed is unrelated, everything else works at the same speed.
- Reinstalling the app repeatedly, before testing on mobile data. If the service is down, reinstalling changes nothing.
When everything works except one thing, speed isn't the cause. Four routes can fail independently: name translation, blocking, a closed door, and packet size.
The shape of the failure points to which. Instant failure means DNS or blocking; a long wait then giving up means ports; partly loading with missing images and video is nearly always MTU.
And before any of it: try mobile data. Half of these cases end there, with the conclusion that nothing needs fixing at all.
