Turntable set up HELP


OK, I have majorly messed something up and need expert advice. I set up my turntable a few months ago, and had no real problems, but I wanted to take it to the next level so I bought a Walley Tractor (well I ordered it in October, got it in January) and I tried to use it today. Well after I adjusted the cartridge using the Wally tractor, re-adjusted the needle force and anti-skate and put on a record. Well about 2 minutes in the record started to skip. Adjusting cartridge force again, anti-skate nothing helps. All the records skip. Platter is still level, I can't figure it out. Took everything off, re-set up, still skipping???

Help!!!!!
macdadtexas
Make sure the arm is clearing the lift platform. Sometimes these develop an air bubble and don't go all the way down and the arm hits against it. If you changed the height of the arm you might need to adjust the lift mechanism also.
I bet mofi got it. I installed a new cart and it was smaller than the last one. When I lowered the arm it hit the arm lift just before it hit the record. I had to lower the arm lift just a bit and all was well.
Mofi and Tfk, thought that was it, but it is clearing. Here's another bizzaro bit to this, I can play regular vinyl but not 180g???
Is the back end of the cart now hitting the surface of the vinyl? Its clearly something about vertical relationships if you can play thinner records OK>