While we're discussing feet...
I broke down and ordered (from Amz but Ebay has them, too) the Box o' 32 feet for $17.99, a couple of years ago. First off, NO felling foot included in the set and the needle on my Kenmore/Janome just barely touches the backside of the opening of each foot. Meaning I will need to file the needle area on any foot I should choose to use.
I haven't tried them on the 15 yet. We'll see. But my Kenmore is quite generic and rarely temperamental. It even uses class 15 bobbins - any make. Has a drop in bobbin with a clear plastic cover, but came with metal bobbins. Thought those were silly 30 years ago, as they blocked my view of the bobbin thread and bought some plastic ones. Yep, that machine sewed just fine with class 66 bobbins for 3 decades.
Didn't catch my mistake with the bobbins until those cheap feet didn't work. Changing to class 15 bobbins didn't help the feet any, either.
I have bought a generic teflon foot from a local sewing machine shop and had no problems. So, it's the cheap feet, I think.