Power being used AC or DC and voltage?

Is your playfield a full 1/2" thick?
How are you powering the coil?

Digging out the playfield would give you less contact with the ball, not more, unless I'm not understanding what you are saying.

Are you using a pop bumper relay like most original pop bumpers? Have a look at MarkG 's web page here
https://www.funwithpinball.com/exhibits/small-boards#PopBumpers

Can we see what your model looks like? As mentioned above, the momentary spoon switch contact needs to energize a relay to power the bumper until the end of stroke switch is opened.

Thanks for the mention @howardr.

Playfield thickness could be a factor as others have mentioned. The gap of the pop bumper skirt, or spoon switch might also affect how lively the pop bumper seems.

A pop bumper relay can be important as HowardR mentioned. The duration of the solenoid impulse will be variable and likely a bit short if you're not using a relay to drive the pop bumper since power is cut immediately after the ball leaves the pop bumper skirt. When used in conjunction with a relay, the relay sends power to the pop bumper when the skirt switch closes, but keeps power pumping through it until the pop bumper end of stroke switch opens. So you get a consistent pulse every time equal to the time it takes the plunger to travel through the solenoid.

Another potential issue is the structure of your demo. When the pop bumper solenoid fires it delivers a pretty substantial amount of energy. But if your plywood can vibrate much a lot of that solenoid energy goes into shaking the plywood, pop bumper and whatever else it's attached to rather than delivering that energy to the ball. If your model is very rigid and doesn't allow things to shake much when the pop bumper fires more/most of the energy will be delivered to the ball making the whole mechanism seem more lively.

/Mark

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.