Perhaps your pooch simply isn't into Yoga! Just kiddin'!
Has anything particular happened? Have you tried anything else rather than Yoga?
If you are really into exercising with your dear furry friend, it just takes some time till two of you find the activity that you can enjoy doing together, in your case, work out!
I'd heard that some Yoga lovers would exercise with their pup, usually small breed pup. But I'd never heard a dog does Yoga. I think the muscles and joints of dogs are tough and stiff, varying from the cats, so they could be hard to finish the motions that need a good flexibility. Overexercise could do harm to their bodies.