If gate goes up, I'm assuming you hit the castle enough times to cause it to do so. This means the game is waiting for you to make it through the open gate to the back of the castle (to blow it up) and won't register any more castle hits until this happens. You, of course, can't do it because your gate has fallen, but that would explain why those additional hits aren't registering anything. Bottom line is that sounds normal if the conditions are right, you just need to find out why your gate is prematurely falling.
No matter how much vibration there is, it shouldn't be overcoming an energized solenoid.... Is there any consistency to what causes the gate to fall (are you sure it's movement, or is there a switch tripping?). Miswired/swapped optos and switches cause confusing behavior back there, but I think you'd have DMD/program clues to go with this. I'd make sure there's nothing mechanical or electrical wrong in the gate mechanism (you sure the solenoid is good? Will the gate fall on its own eventually without any vibration?).