Help - Bobbin not catching ONLY when sewing leather - Singer Heavy Duty 4452

I'm working on some test swatches for a project that will use 10.10oz canvas and (pretty thin) genuine leather (excavated from an old jacket). I'm using bonded nylon thread (69), and I have a Singer Heavy Duty 4452. I'm having no problems with the canvas, but as soon as I run the leather through the machine (either alone, with canvas, or with batting) the bobbin thread simply won't catch.

Sometimes it'll catch for about 1 in 10 stitches, and then inevitably stop catching again or get tangled beneath the bobbin plate. I've messed around with top thread tension and bobbin tension for several hours now, and still struggling. Any ideas would be appreciated! Thank you!