Drawing the plunger...does it stick and not go any further, or does it pause then continue passed the imperfection, then pause again as You push the plunger back out/forward? If it stops completely, it may be a nick, or maybe the tube is slightly caved-in, not completely round? If it's Out Of Round, I'd contact BT for a replacement.
I'd pull the plunger out and check the interior surface, maybe with a Pencil Eraser or something similar (something that won't scratch aluminum)...long enough to reach into most of the injectors length. If You find a nick or scratch, maybe hit it to try to "knock it down" if it's a nick...or again, contact BT for a replacement.