my 107 year old basement floor is hardly flat, so I have so many issues with unevenness. Every pin that comes through the door gets new levelers before it even gets powered up.
I get a friend or the wife to use the level on the playfield while I crawl under it and use my back to push the machine up for adjusting.
My most recent pin, Gorgar, had rusty levelers. I used WD40 and ended up using a hammer to get the old levelers straight because they were tweaked from straight. It might of helped to get them loose. I would seek out suggestions from a local car repair place on a good penetrating product, like others have mentioned, for really stuck on levelers.