For me, it's getting some devs to become team players. Many of them are good at what they do but sometimes they become big blockers to the progress of others in the team. I try my best to unblock people but you just can't unblock someone's attitude to working. Maybe they just need time to understand, but time is also a luxury our projects do not have.
I've encountered my fair share of such people and I probably was one of them for at least some portion of my own career :(
Is there anything in particular that worked for you?