Do part time union workers have more seniority over full time workers?

I am a union delegate at a shop in NYC. For some reason I can't get a straight answer from my business agent. There's a (A) list part time worker with us, he's the 3rd in line out of 8. Even though he's PART TIME and has another job in the same union, management gives him right over 6th and 7th days (which are all overtime) over us full time guys! So we lose out on money. Director says it's because he has seniority... Any union workers' input????