Why XP is not shared after submitting 1 already finished quest in group?

I wanted to help out a couple of my buddies on borderlands 2. I'm op8 and they are level 10's and 1's, the way I was planning on helping them out was by turning in a whole bunch of quest I never turned in for this reason alone, the very first quest I turned in they got experience but every single quest I turned in after that one they didn't get xp from.

Can someone help me figure this out?

Mm, yes. Borderlands has always had this system in place. After turning in the first quest, Borderlands realises that the others haven't done them.

If you want to level them up quick, just do Pyro petes bar OP8. It'll work magic.