Sometimes you can dry the skin out too much with an IPA prep if that's what your using. Especially if the skins already dry and dehydrated. Will make it more difficult for the blade to slide over the skin, even if you have good skin tension. Try doing a deep cleanse first. It sounds counter intuitive because they say try to keep the skin dry for traditional dermaplaning. But the hair can turn too brittle, tough and just stick to the skin instead of getting chopped off. Try going in different directions as well. Hope this is making sense lol... You can also try wet dermaplaning with coconut oil, takes a bit of practice. It may also be the general health of the client as well, including their medications. Lots of variables at play. I'm afraid it's just a case of trial and error.
Hope this helps.
Nicky