If it looks like data is not synchronized, usually the first thing to check is the network connection between the devices. Usually it's helpful to start the Sync server with HTTP enabled, so you can e.g. check the connection using the HTTP server URL in the standard browser of clients.
The network connection seems fine? OK, let's get additional information! The Sync server comes with a switch to turn on debug logging. Logs go to standard output and are typically very sparse. Debug logs on the other hand provide you with a lot of information. Once you get used to the amount of information, you will learn to identify problems. For example, a client got disconnected? The debug logs usually tell why.
A checklist of other likely issues:
Do you have the latest versions running?
Does the server have the latest version of the data model?
Does the client version "match" the server version? This is fine most of the times, unless we announced breaking changes during a beta phase.
The ObjectBox team is here to help you. If you already investigated a bit (e.g. "hey, this debug log there looks odd, no?") it will help to get issues resolved quickly. In any case, do not hesitate to reach out! ❤