When a dog responds slowly or refuses to respond to a command he knows very well and generally responds to enthusiastically, or grumbles/snaps when touched or handled, he may be in physical pain. It is important to have one’s dog thoroughly examined by a veterinarian before undertaking any dog obedience training or problem-solving.
I once had a boxer that was very reluctant to follow the command “sit”. Even she knew the command, she refused to obey, or seemed to be at least very reluctant. When taking her to an animal trainer the problem got worse, she even started limping. A vet determined that the (young) dog was suffering from osteoarthritis and obviously the sitting down caused her pain.