You can remove hands from PokerTracker by using the
Purge functionality.
However, you should know that this will remove all stats on your opponents from those hands as well. If you just want to see data on yourself that is more recent you should consider using our
Filters interface which you can configure to only show you recent data - then the old data is there, just hidden.
Regarding your missing data - do make sure you don't have a filter enabled to block those out; click the Clear button at the top right and that may help.