Re: The plastic tip at the end of the primer tube
DO NOT over-tighten the knurled ring at the top. Doing so will twist the plastic tip at the other end and you either won't get any primers being fed or you will get lots of primers dropping out at the same time. You just want it slightly past first contact, and not snug or tight. Also, be sure the plastic tip is keyed in the slot before you tighten the lock nut.
Next, be sure the rubber (brake line) tubing (P/N 14990 Slide Roll Pin Sleeve) is on roll pin and that it isn't torn/worn.