Scraped Knees from Push Ups, Planks, etc


Every time I do push ups or planks from my knees (I can only manage this type) I end up with scraped knees. I do them on a yoga mat, but apparently it´s not doing much for the tender skin on my knees. I have also tried a folded towel to provide a more cushion surface.

What am I doing wrong? Anyone else with this issue?