At some point, everything stops working; Find declaration, Error insight, etc.
When that happens, try the following. I am not sure it will work for you, but for me that was the cause of so many aches that I made an expert that does it for me. Try this:
1- Delete ALL .DCU files that your project or the DelphiLSP can see. For me, that was *.DCU in my project directory.
2- From the tools menu, restart the LSP server.
3- Wait 5-15 seconds.
For me, this solves the problem. Note: a "Clean" does not fix the problem for me because there are always some DCU file left.
I found out that when the LSP server encounters some DCU file it completely stops working. And since a lot of the IDE functionality is based on the server, a lot of things break.
Very janky workaround. This has been going on since Delphi 12. Someone at Embarcadero told me the disabling of "legacy" code in Delphi (refactoring, find references, etc) will allow them to rebuilt the features better and improve the LSP.
Right now, the sad state of release 13 prevents me from using it.
HTH!