Your old hand history files should get moved to your processed folder when you stop getting hands and if they don't get moved your hand history folder can accumulate with hundreds/thousands of files which can cause problems with auto importing and the HUD. To check your settings go to "Configure -> Site & Import Options -> Preferences" and make sure "Move Processed Files To:" has a check mark. The default folder for moving processed files is this:
C:\Users\{Windows Login}\AppData\Local\PokerTracker 4\Processed
If that's setup correctly and the files are not being moved then please
enable logging, reproduce this problem and then send your log file to us in a
Support Ticket after you have stopped getting hands. You can access the log file by clicking 'File --> Show User Data Folder' in the main PokerTracker 4 window and depending on your Windows settings it will be a text file called 'PokerTracker4' or 'PokerTracker4.log'.