If you are having problems seeing stats on a 6 max or Heads Up table, then you probably need to tell PokerAce Hud how to treat the table.
This is done by right-clicking on the table name in the PokerAce Hud table list. A menu will appear that has the options you can choose. Choose which table type it is, and your stats should appear correctly.
Also, make sure that PokerTracker is importing the hands for the correct table size. For example, if you are playing 6max, make sure that the table is being listed as 6max in PokerTracker. If it is not, you will need check "Treat all hands as 6max" in the auto-import window.
If once you do this, your stats disappear, check your filters. If you have "Show Stats From Current Limit Only" turned on, you will need to do one of two things. Either turn that filter off, or go into PokerTracker and tell it which sessions need to be 6 max.
If you have followed all this and are still having problems, then make a post. Please explain that you have read this post and things still aren't working.