The arm may be Weak because your shoulder position is slightly off.
You may be able to get one of your Bros to watch you bench, and see if you're shrugging or rolling that shoulder, or doing something weird.
Also, rows. Make sure your shoulders are level and doing the same thing when you row. You might even get your Bro to poke you in the mid-back, to make sure that cranky shoulder is actually doing what you think its doing.
Take a minute to make sure things are right.
This stuff is really boring and whack. But you're a young dude, so you can fix things quickly. If you just ignore it, then do 10, 000 poor reps in the bench press, it's going to be very, very hard to change.