Gotta be mechanical.

Pull the front rail that holds the button off the game, does the button stick only when it is pushing on the switch? This might be because something's out of alignment. Loosen the nut on the bottom of the rail and re-seat everything until the mechanical binding is fixed.

Or is it sticking even with the button and rail off the game?

I've had to replace the button, part # 515-7791-00 for about seven bucks plus shipping. Marco and Pinball Life have them.

If it is sticking because of soda, unscrew it and soak the transparent part in soapy warm dish water then rinse and let it dry.

No game comes from the factory without problems.

Hopefully you bought from someone local who can come over and fix your problem.

If you are your own repair person... well, it's gotta be mechanical, and the above post might allow you to figure out and fix it yourself.

You've been here for 11 years and have 16 machines, you can probably figure it out in 30 seconds if you look at it.

No magic here, it usually hangs up for two reasons -
Switch needs adjustment and it is catching the bottom of the button - like an eos that caches the flipper mech
Button shaft needs sanded/smoothed - sometime a plastic burr or something isn't smooth, similar thing can happen with ne flipper buttons.

Nick, this happens on my last 2 Sterns. The trick is to hold the button in as you're installing the lockbar, making certain the tip of the button's bottom is squarely on top of the white switch nub (don't know the technical term). once it's locked in place, release the button and it'll be good until the next time you remove it.

** DON'T go sanding or trimming anything!

This. 3 of my last 4 Sterns have done this.

So it was the switch.

Sorry I came off harsh, just a bunch of people posting questions lately without taking the time to remove the glass and look first.

Same here. After installing many mods I now managed to put it in without properly aligning the tip to the white switch nub and something must have been bent in the wrong way. Since then the action button doesn’t work anymore even after proper alignment. Are there any good instructions on how to fix the mechanic? Or is there a part that I can order to replace? Thanks for any advice.

Take lockdown bar off. Go into Tests - Switches - does it register ? If so arm on switch needs adjusting so the button when pressed closes the switch.

Doesn't work in test - check for broken wires and with game turned off put a meter on the switch and see if it works. If so then broken wire(s) from switch to board.

If switch is dead - https://www.pinballlife.com/switch-0365-17032-0009.html you might have to trim the length of the blade on the switch.

LTG : )

Thank you

It's just the leaf switch being slightly out of position when installing the lockdown bar. The edge of the leaf switch is getting stuck on the grooves of the button of the button.

Can either simply move the leaf switch and center it when installing the lockdown bar, or get under there and tighten it when centered so it doesn't move the next time you have the lockdown bar off.

OMG! So simple and so HELPFUL!! Have a few games (Deadpool and Mando) this has been an issue on and this procedure cleared it up! Thank you!!!

I’ve removed my lock down bars 100’s of times on my route games and have not had these issues. If button getting stuck you need to adjust the leaf switch. Raise playfield to up position and put the lockdown bar and see how the button is interacting with the switch.

No freaking way. This literally fixed my machine. I tried bending the leaf switch, removing and reinstalling the button. Nothing worked. I took the lockdown bar from another Stern and it worked fine. Finally, I found this three, held in the button, installed the lock down bar and it worked on the first try.