Problems with U-coupler

[TTFireman]

I have the micromatic U-coupler with the ergonomic grip. I've used it on two 50L Guinness kegs so far. Both times it gets stuck so badly that I have to have another person help me tap and untap the keg. Even with a full 50L keg I have to have someone, usually someone besides my wife because she isn't quite strong enough, hold the keg still while I turn the coupler all the way. It will turn 3/4 of the way, but then gets very difficult. I am a big guy, and I can tell you we are having to use A LOT of force to make the coupler seat enough to lower the handle. Actually, while removing the coupler from the last keg, I unscrewed the top of the spear from the keg. It came off with the coupler and I had to remove it from the coupler and screw it back onto the keg.

Is this an issue common to U-couplers, or could I have a defective one? I have zero problems tapping a standard d coupler keg, so it's (probably) not operator error.

Thanks,
Ty