this could be a whole series of issues including hook setting (timing).
What size needles are you using? 65-11 should be the minimum.
what happens if you start a design on one good head and one bad head sew it partially through then just for giggles switch the hoops over to the other head. The issue could be in your hooping
Your heads could be off register, check this by placing a piece of tape over the needle plate and rotating the shaft by hand or on a trim command on needle #1 to penetrate the tape just once.
Switch to needle 8 and re-peate the process the needle should use the same hole created by needle #1 then again on needle 15 . If any of the successive needles alter the hole location / size the heads are not in sinc or off line and need to be adjusted.
Needle depth could also cause this issue, you should have a set gage with your machine.
What speed are you sewing at, typically 450 to 600 is the usual for caps.
There are other possibilities but you can check the above issues in a couple of hours on the outside.
mooseman