This is a relatively new habit, but it's getting right on my tits. When I train Jasper, I hold the treats in my hand, give the command, praise and treat (standard stuff). In the last few days, though, he has started barking at me if I ask him to do anything requiring effort. So, sit, paw etc no problem. But rollover (which he has been doing for weeks) suddenly earns me a 'woof' and a lunge. Sometimes he mouths my treat hand, clearly thinking 'Bollocks to this. Just give me the goodies'. I take away my attention when he does this, and abandon the session. Can anyone shed any light on what's going on? And am I reacting appropriately?