It seems a lot more information is needed. I have just started to inject and have noticed some air once in a while, but hotter plastic and a little more consistent pressure took care of it.
The other thing we all need to help diagnose is the products you are using. What plastic, injector, colors etc... what are you using to heat and stir the plastic. This is somewhat like forensics where we have to know almost everything to get an ahha moment.
Kenneth