Maybe this explanation will help. For clarification, my armor is FettPride V3 connected to a BatNinja vest with the stud and grommet method. I use a BooBoo Fett aluminum harness.
My back plate has two bolts permanently affixed on either side of the "shoulder" portion that poke up from underneath where the shoulder studs will eventually be. My collar armor is connected to the vest with two studs through grommets located underneath of the two forward most angled portions of the collar. The rear of the collar is completely free floating from the vest and has two holes on each side of the rearmost portion that line up with the bolts on the backplate "shoulders."
After donning the vest with the attached collar armor, I have my assistant/helper slide the backplate on. The two "shoulders" of the backplate fit underneath of the collar and line up through the holes at the rear of the collar. My assistant helper then screws on the four shoulder studs to affix the backplate to the collar armor. So the layers from bottom to top in the shoulder area are: flightsuit, jetpack harness, vest, backplate, collar, and studs.
This arrangement means that the backplate is only connected to the costume via the collar at the four bolts underneath of the shoulder studs and the rest of the backplate hangs free. If the position of the bolts on the backplate shoulders is too far back, you can end up choking a bit even before the weight of the jetpack is added.
I hope my explanation is clear of the causes. I will offer solutions that worked for me next.
