Yep mine wanders off too, if he could put up his paw in a 'talk to the hand' gesture then he would. When he is hungry and smells liver he is concentration personified all will do all manner of stuff. Fickle little sod!
I would feel great about tiring him out, but it literally lasted about 2 minutes, lol
I was trying to teach him take it...I presented him with a toy, he picked it up I clicked, he spat it out and took a treat, I tried a few more times, he seemed to think I was doing drop it (fair enough I can understand the confusion), I waggled it a bit, he bit it and looked at me expectantly, I waggled it a bit more, he looked at me like I was insane, pushed it towards me and went to sleep - I'm pretty sure he said do it yourself then, lol
Norty Minx does this sometimes at class - in fact she did it tonight. Just flicked the switch and needed a lie down quietly for five minutes. Then after that, she bounced back up with the twinkle in her eye ready for more adventure.