About a week ago, I was trying to sew some light cotton fabric with a ¼” seam on my Singer 31-15. The stitch would progress fine for a while, then the fabric would bunch up under the presser foot. Thinking the thread was getting caught somewhere under the throat plate, I tightened tension, increased presser foot pressure, checked for burrs in the hook area and everything else I could think of that would cause the thread to get caught and stop the fabric from feeding.
Almost by accident, I saw that one of the legs on the throat plate was broken away from the plate. When the presser foot was lifted, the leg came up into its natural position; when the presser foot was lowered, the weight of the foot pushed the leg down, leaving a rough surface on the plate itself for the fabric to catch on. One theory for the cause of this is that, in the past, the presser foot has been allowed to freefall onto the throat plate, rather than being gently lowered.
This not the first time I have had this problem, and that’s why I mention it here. I got a Necchi Lycia off ebay that had exactly the same issue. The machine worked very well, but fabric kept bunching up beneath the presser foot. Parts for most common industrial machines, like this 31 Class Singer are easy to find and reasonably priced – I got a new throat plate and feed dog set for $4.95 plus shipping. I don’t remember if I ever did find a replacement throat plate for the Necchi.