There is a precedent for this you know - in the very first movie, the Alien ignores Parker at first even though he is obviously a bigger threat - walks right past him and turns its back on him. Now don't claim the Alien didn't know Parker was there, it knows where a cloaked Predator is for crying out loud. Basically Aliens focus on one target at a time, whether to kill or drag off for cocooning. They probably don't know what a plasmacaster is so if they see one of their comrades fall due to that weapon, it probably would take a minute for them to figure out what is going on and where the threat is coming from and then respond accordingly. If that theory doesn't fly for you, please explain why the Alien walks past and ignores Parker.
