I learned something last night about a machines ability to sew through heavy or uneven stuff.
I was working with a Singer 413 I'd just cleaned and lubed and was sewing on a small piece of denim. I make a little pad from denim scraps to go under the presser foot for each machine I have. Two pieces of denim with the edges of the larger piece folded over the smaller piece and stitched down. This machine would not feed the uneven layers of denim. It just quit feeding.
I grumbled that even my nearly 100 year old treadle would walk over that like it wasn't there. Then I noticed the presser foot was cocked at an angle and acting like an obstruction. I swapped that out for a different solid ankle foot and the problems went away.
The original Singer snap on ZZ foot was the problem and will be replaced with a good solid ankle foot as soon as I make my next Sew-Classic order.
We've also noticed this with our 401A too. The aftermarket and Singer solid ankle feet work better than the snap on feet. I've decided to eventually replace all the snap ons. They just don't work as well as they should.