A couple of good resources on this in addition to those I've pointed to previously:
and the explanation and video here of the Cook Hip Lift. That one identifies something that really applies to me - when I do a one legged glute bridge with the right leg, it is my hamstring that cramps.
Stuart McGill calls this inability to use the glutes "gluteal amnesia". The brain has literally lost touch with the glutes, so to speak. It could be that the psoas are "tight" and so it is a case of reciprocal inhibition. Or, the glutes are weak and so the psoas are tight. Heres the thing: It doesn't matter. Turn on the glues and solve the problem.