If shear pin was broken you would hear spinning sound and jack would fall to the ground. Here's a link that has been posted a couple of times on a lot of good information concerning electric power gear jacks
http://www.irv2.com/forums/f107/power-gear-jack-problem-53137.html(The last couple of pages is the progression to the solution of seal, seal, and seal some more)
Fifty pages is a lot, but to sum it up. You need to seal the jack to prevent water intrusion to the brake. This is the boot and the top of the jack casing, as my jack brakes have failed I have removed the jack and sealed it completely. Until I completed this process the system could be very frustrating. So far with the jacks sealed they have worked flawlessly, now I have probably jinx myself. Good luck