If you don't touch anything to the under bed mechanism, in my opinion it could be the the needle bar height. I saw some 28 machine where it was impossible to adjust something, but on 127 and 128 it's possible. You could see how to make this adjustment on page 9 of the adjuster manual. I think also that you clean the needlebar when you removed it, I had a similar problem on a 28 and it was a ring of old gummy oil that cause the problem.