It is quite likely that the machine needs oiling. Somewhere in the drive train parts that need to move freely are sticking and causing the stitch length lever to drag back to a shorter stitch. Use a good quality sewing maching oil on everything that moves. Preferably use singer oil. under no circumstances use a "General Purppose oil". Machine oil is designed to evaporate leaving no residue behind whereas general oils simply dry out leaving a horrible brown varnish that actually gums up the machine. If your machine has brown varnish everywhere you look then this is probably the problem. You will need to thoroughly clean the machine with paraffin oil (kerosene) and then thoroughly oil everything as above.