Off course you have released the clamp? Once you get it lowered, "exercise it" frequently just like your through hulls. The best way to keep it from rusting in place is not to let is set in the same place all the time.
You might try pulling it up carefully (grab the clamp handle and rotate a little as you lift). If the counterweight is stuck as Rich suggested, this will give some "slack" for the weight to dislodge and release as you tap the compression tube.
I don't have a problem with lowering or raising mine with this routine.