I had the same problem with my chessy. I just backed off for several months (actually new job, new baby, and moving was more of a reason but anyways)and tried again after some time. He got it the second time around after roughly 3 weeks of working. 10 min. or so a day, some were good and others were terrible. Then it was like a switch was flipped.

Buster would lay down with a bit of pressure, so I had to keep heeling him, sit then hold. I had to physically hold his jaws shut. When we were around the 10 min mark or he seemed to catch on to the concept for the day I would kennel him until the next day.

Just be patient and strict. It was a true pain in the begining. Nothing like wrestling a hot stinky 95# dog. I just worked on the ground. Had a rope strung head high and just attached his leash to it for a bit of extra control when I needed both hands, legs, and gut.=)