Why does Signaltap insist on recompile after recompile?

Hmm. You have multiple instances. If you disable the instances that it says are not compatible, does it work?

There might be something in those particular instances that is causing the issue. Perhaps you're trying to tap post-fit nodes that no longer exist after some change you made a while ago. Check those instances to see if you have any red signals in the node list. If the tapped nodes can't be found by Signal Tap, it will indicate you need to recompile to connect to them. But if they don't exist, it still won't work. This could be the problem.