I use a Kenmore and just did a set of tree straps this AM without a problem.
Ditto on what the others have said about replacing the needle
One other thought...when was the last time you oiled the machine. It can make a big difference.
Maybe try a smaller needle since most tree strap material isn't tightly woven a small needle might make it easier to penetrate. Also make sure it is a regular point needle not one with a ball point .
Glad to hear you are sewing (a few of) your own straps. Almost certainly a "Kenmore" (or Kenny" from 1945-1975 can do this with the right and good needle and the right and good thread. There's enough power in the motor to drive the needle and feed once it is going. But, there are dozens of different Kenmores, and there could
Likely the needle, but you could also have a way-too-tight, feed, squeezing strap between foot and needle plate. But the problem description is too vague. You rule out faults by trying other materials, needles, thread, and settings.
The denim needle is not the easiest one to drive. It is designed to not break going through multiple layers of denim where the seam meets the hem --applicable -- and also not break climbing up to those multiple layers-- irrelevant to strap sewing.
Use strong and heavy polyester thread for straps. Thin thread deteriorates much faster and may tear out sooner. Calls for a 16-20 (90/120) needle if that will fit in your machine. Maker of thread DOES NOT MATTER.
Good tips here, even for other types of sewing. Thanks for the advice.