Please make sure that that the option to 'Save My Hand History' is still enabled in PokerStars (Settings -> Playing History -> Hand History) and that your folders are still correct:
PokerStars Configuration GuideIf everything appears OK then
enable logging, reproduce this problem and then send your log file to us in a
Support Ticket so we can find out what's causing that. You can access the 'Logs' folder with the log file by clicking 'File --> Open 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'.