Clipper blades aren't really meant to go through mats and knots. Sounds like the #10 blade is short enough to go under them, which is why it's working better. Without brushing out your dog's fur first, I don't know that any long blade is going to be able to get through a coat like you describe. Clippers generally work best on brushed out, washed hair or fur, regardless of the species they're used on.