I think I know what you're talking about, I had the same experience when I built my first custom pinball machine. Take a look at Bobs video of his homemade pinball machine, are your bumpers behaving similarly to his ?

If so, the problem is the already mentioned activation time of the bumpers which is too short if you just use a "short circuit" to activate them.
I solved the problem by activating my bumpers with a microcontroller, in this case an Arduino. It's very cheap (Arduino + materials are under 10 bucks) and reliable and maybe also interesting for your project depending on what you want to include.