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Agree completely with what Mick said, as usual. Take a break for a little while, then keep the sessions real short. Also, on club training days, I never feed the dog in the morning before going to club, only in the evening after club. An empty stomach is a great motivator.
A guy at our club had a dog that was getting lazy in the bitework. This is what he did and it seemed to work: He brought the dog to club, brought him out JUST to watch another dog doing bitework for a few minutes...then just put him back in the crate. He did this for about 3 weeks. Then he ONLY worked the dog on the pole for a minute or two, with no actual bites allowed on the sleeve...just a couple frustrating misses. Seemed to help this dog a lot.
When Shane got a little lazy on his tracking last year, a good high level trainer told me to ONLY feed him his regular meal at the end of a successful track. Didn't take long for him to REALLY want to track.
Keep the OB sessions really short, and try to end on something really good and successful. I have a BUNCH of different balls and small tugs, and I try to use a different one at the end of obedience at the club for a reward. For some reason, Shane goes nuts when I take a ball at the end and bounce it off the A-frame wall for him to chase.