That's one of the worst places for a mat because it's a sensitive spot and my cats also get very, very upset when I try to get clumps out in the same area. Have you tried teasing a little at a time with your fingers? It takes a long time, but I do it while I am stroking them, just slide both of my hands down and give a little tug to the mat from either side. I usually get about two tugs in for each fussing session before the cat gets so mad that they retaliate, then I give them lots of regular fussing so that is what they remember.