I went to a handler course this week-end and asked almost the same question. Only I asked how can you keep a good topline on a dog (cause one of my dogs sometimes loses it as well).
I was told that the more excited (not over excited..) the dog gets in the ring, the more firm the topline will be, because of the contaction in the muscles.
So perhaps he's just bored in the ring
I've also been told by another breeder that you're able to make a better topline by letting it run along your bicycle, at a gallop not at a trot.
But it's a very interesting question and I'm waiting to hear an answer