Possibly magnetized plunger? The next time it sticks, turn off the machine. If it's still sticks, then it's mechanical. If it drops, then it's electrical.

Would be worth plunging it manually to see if you can get it to stick. This will also tell you if it's mechanical.

Have you started with the basics first?

- How old are the sleeves?
- Any mushrooming of the coil stops?
- Are the stop lose? This is a very common issue on Stern games and will cause slicking issues
- Is the return spring properly tensioned?
- Hows the flipper to Playfield bushings?
- Hows the gab between the flipper bat and the bushing that goes through the PF?
- Any slack in the plunger mech (there are two bushings their that wear out)?

Looking at and resolving the above will resolve 90%+ of sticking flippers that occur. I don't think that I've ever encountered a magnetized situation on a Stern flipper. If it's slightly magnetized the return spring should still overcome that.

+1,Try doing this

Grease?

There should be no grease anywhere in most, if not all, mechanisms. A flipper rebuild kit is much needed. If ovoid in shape, i would check the bore of the flipper coil as that means the coil overheated and warped. A new sleeve will not fit into a misshapen coil bore.

Correct. If the sleeve slips in fine it's fine. Sometimes you have to drill them out to get the sleeve easily in.

No grease in modern era pins almost anywhere....but for sure never flippers. Anyway, just sit back and wait for new flipper rebiuld kits. Oh hey...did anyone tell you to order new bushings as well? They usually aren't in the kits I don't think

Take ALL the grease off, and make sure it's off. Use a degreaser if needed.

Rebuilt the right flipper, I think he should've said I pumped it full of grease to keep it working.

Ouch, sounds like a temp fix just to sell the machine. Sorry that happened to you.

Glad you found your culprit. I know it can sting but I would strongly suggest doing all 4 flippers at the same time. That way you know your game is solid and will play great for a long time coming. But at min do both the bottoms. Doing a flipper rebuild on just one could cause the flippers to perform different if the other flipper is not working 100% as well. Due to the preceived neglect from the previous owner (putting grease in there) those full rebuild kits will be the best bet now. Order a couple extra coil stops and sleeves though as those two items seem to be the ones that wear out the quickest and are fairly cheap.

Those kits don't come with two important pieces that should also be changed.

Flipper bushing - https://www.pinballlife.com/index.php?p=product&id=271
Spring Washer - https://www.pinballlife.com/index.php?p=product&id=279

I know the first rebuild can be a bit intimidating but it's not to hard if you take your time. The only real important thing that wont be obvious when doing this is setting the flipper gap between the flipper bat and the bushing that goes through the PF. A lot of first timers set that clearance to tight (push the bat all the way down). You should have about a credit card thickness between the bat and the bushing. There is a tool for that (http://www.planetarypinball.com/mm5/merchant.mvc?Screen=PROD&Product_Code=PPS-03-8194) but you can easily make one with an old credit card or piece of plastic the same thickness.

If you get stuck and need help/tips post them up and we'll do the best we can to assist.

Between coil stop and coil.

In this order. Coil Stop - Washer - Coil.

Helps keep coil tighter in the bracket.

LTG : )

Correct, not at this time anyways. If they are flipping OK and just sticking then I'm betting coils are fine.