It’s likely they’re moving too much (too much friction). This happens when we’re using our low backs a little too much to do the work: you ‘scooch’ during each rep. Try to make sure you’re pressing your low back into the bench/floor throughout the entire move: it should stay stable throughout. This may mean you’re not able to get the same range of motion, but you’ll be using more of your core, which means better results overall. Gradually sink lower into the moves as you get used to proper form. You may also benefit from placing a mat instead of a towel there, or even a pillow.
Another option is to get off your back entirely: swap in planks, knee raises (hanging from a bar, lift your knees to your chest without swaying. Straight legs is harder), tuck jumps, mountain climbers, burpees and more to work your core. They tend to be more effective anyways, and you’ll avoid that burn by getting off your back.
Hope this helps!