Is There a Reason Tuck Rhymes With F....?

I tried to swatch in tuck stitch again on the Singer 155, and the stitch selector would NOT move into tuck setting. Not, "it's stiff and needs lube" feeling, not "if I push it will going sproing or snap", just nothing. It just wouldn't go!

I took the cover off again, that little piece that bent before is still okay. The stitch selector cam just won't go past slip stitch. I took that off, and underneath, at the bend in the groove near the end that I think means it's in tuck, it's been worn. When I turn the carriage over, I can move the levers and flippers freely. When I try moving the plates under the stitch selector by hand into the tuck position, it's very difficult....a lot of resistance. Not stiffness, but it's like pushing against a spring. Should it be like that? Everything else works fine, although I do check after putting it in slip stitch that both flippers underneath have moved; it was slipping in one direction but not the other for a while. But I don't really like slip stitch. It's neat for Fair Isle look, in that there aren't floats, but it's a pain to change the yarn every two rows (in Fair Isle, both yarns are in the carriage all the time). I did cheat at the tuck, by having it in slip stitch, and manually putting the float onto the needles, but I think the float in Tuck stitch is a bit looser. I would like to be able to do tuck stitch---I can do it manually on the other machines, so it was nice to get a punchcard machine to do it for me :)