A new yoke O ring will take a few dives to get "seated" (meaning stuck) in the orifice. So if you blow one and replace it with a new one, and don't fully depressurize the reg, it will blow a lot easier than one that has some dives on it. The good news is, O rings are cheap and there's nothing wrong with having a new one every dive. 
Do you do that silly blow-on-my-first-stage-with-the-tank thing when you take off the reg? That can dislodge an O ring, too.
And DIN O rings do sometimes come out, too, and are left behind in the valve opening, but not as often.

Do you do that silly blow-on-my-first-stage-with-the-tank thing when you take off the reg? That can dislodge an O ring, too.
And DIN O rings do sometimes come out, too, and are left behind in the valve opening, but not as often.