The 331 is similar to a 335 in the fact the feed dog doesn't lift the material off of the needle plate when it goes back.This creates alot of drag esp in sticky materials like vinyl & leather & will cause the length to shorten.You could have some worn feed parts,lift the foot up & push the needlebar front to back & try the feed dog too to see if there's any play.
The 331 a needlefeed & looks like the 335.