Power Door Locks

[treeperch]

I have a 2000 GMC Sierra 2500 4wd crew cab that I cannot figure out the door locks on. When I push either switch to lock or unlock the doors, all four doors will "click" but nothing moves to lock the doors....about one out of 20 tries, the driver door only will lock, but then it will not unlock.......or if I lock them all manually and try to unlock them with the switch I get that same thing - just a click. Same thing applies if I try using the remote door lock button. Could this be a bad relay not supplying enough current to properly operate the actuators?