show sheen the ever loving life out of the area
My horse is the same way. I have to clip in several different directions over his rump before I can clip I the direction it's supposed to be done from because its so thick
Make sure you bathe really well there with soap, don't let too much hair get built up in your clippers, and rub rubbing alcohol/water or just rubbing alcohol into the hair just before clipping.
I used a vacum to get all the dirt of my horse but he is bullet proof so that could only work for certain horse but maybe your blades are dull so try cleaning them or borrowing someone elses clipper and see how it works