How do you manage if it's impossible to get to the skin?

My burmese has short hair but very thick pelt nonetheless. I frontlined him the other day - I think. I don't know how much if any got through to his skin. The rest of it was on his fur. I've also profended him today with the same problem.

Do you think the treatments will work or should I be looking at other options for flea/worm prevention?