Wow! My 301s are much less fussy than this post appears - try an Elna
. Did you remove the bobbin part that holds the case? The one you have to remove the screws from the top? If you did, that finger has to be exactly in the grooves in the removed plate before you tighten the screws. It will pretend to be correct, but do just what you describe. My 301s sew with any needles and without holding thread tails, but not with that finger out of place.