I had a similar problem with one of my QDs. It was connected and working when I bought the tractor, but I had to disconnect for some reason. Then could not reconnect one stubborn QD. After some consternation, I realized the collar didn't slide back as far on that one as it did on the others. I'm going to assume yours is like mine - where the female half of the QD is attached to a fixed bracket.
Look closely where the problem connector is attached to the bracket. The one that gave me trouble had the retainer clip on the back side of the bracket. That was enough to move the connector far enough back in the bracket that the collar could not be fully retracted. I removed the clip, repositioned the connector, reinstalled the clip - in front of the bracket this time.
Oh, and I keep a 1/2" wooden dowel in the tool box. When a line pressure problem prevents coupling, the dowel and a tap from a wrench or hammer is all it takes to relieve the pressure.
//greg//