I have a theory. If you produce a lot of oil on your scalp and don't wash your hair as often or don't dermaroll, then absorption is limited. I know people like to dermaroll and put minoxidil on right after, but that would be more similar to taking it orally which defeats the purpose. This is why they say to wait 24 hours after dermarolling or thoroughly cleansing your scalp. I also believe this theory from personal experience using Minoxidil for 12 years.