Why Can’t I Sync Messages Between My Phone and Tablet?

I’m hoping someone here can help me out. I use a messaging app (let’s just call it “ConnectApp”) on both my Android phone and my tablet. For the most part, it works fine, but I’ve been having a really annoying issue with syncing messages. Sometimes I’ll have a whole conversation on my phone, and when I open the app on my tablet later, none of those new messages are there… Any advice would be appreciated!