I have seen the debug hover stop working, but I seem to recall that it is usually fine after restarting PL/SQL Developer.

It seems to stop during a debug session; it might be related to hovering over some 'unusual' variables. I seem to recall it occasionally stopping when I was in a procedure using a record with 'bad' column names, like 'TYPE' or 'NAME' or some other reserved or unusual words (hey, *I* didn't design the tables! 8-) ).