I agree with the comments by Norton. I have the same accessory, which needs the same tinkering, but if the car does not vibrate at all, there is a circuit problem. Whenever the car is moved even slightly, I must shift it a bit to find the sweet spot. Once I find it, the doors open and the car vibrates. Now, that does not guarantee the cattle will jump aboard. Some of them need a push from the next in line!
I would suggest to find a circuit issue. Might even take the car off the rails and apply direct voltage to see if it activates the mechanism.
Good luck!
Michael