I have 2 of the 4 oz do-it injectors one brand new the other a couple years old, I noticed the new one seemed like it wasn’t filling all the way with plastic (almost like it’s drawing air from the back). I never had this problem with the old one until a few days ago when I replaced the o rings and now it’s doing the same thing. It’s not that big of an issue but I used to be able to fill 2-3 molds with one injector but now it’s only drawing enough barely fill 1. Any idea what could cause this and how I can fix it?