A Nextcloud Client having issues with the synchronization with the server

I have installed the latest snap of NC Server. (automatic update)
Some clients are having issues to synchronize with the server suddenly.
The users are using the Nextcloud client version 3.3.6. The synchronization is not good between the user and the server.
Sometimes it just stops the synchronization between the user and the server.
Uploading from the web works fine.
Tried reinstalling NC Client, it works for some time and then it just stops.
The files are small. The internet connection at the user is good.
What should I try?
Is there any settings that makes the client check for changes more often?