Try applying a decent amount of force (cracking your knuckles off a pulley kills) and while pulling out on the elbow rock it in a circle. I don't was to say rotate it, because that's what could damage it if you aren't careful, but just imagine rocking it back and forth and turn the motion into a circle :P

Mine got stuck once when I forgot to lube one of the o-rings, and the elbow somehow seated itself at the perfect slant to get jammed. Eventually that worked for me.