I always admire veterinarians more than people doctors. People can tell the doc that they have heartburn, or their bunions hurt, or that they are depressed. A vet, and a dog's best friend have to guess.
Is there an obvious reward for Happy when she finds the deer?
Is she in perfect health? Was she reluctant during weather patterns that were not conducive to tracking? Was it uncomfortably warm?
My dog slows considerably in the summer heat, but an opportunity to chase something always picks her up. Sometimes she gets bored with a game we are playing, and quits, wants to do something else.
You know your dog best, I am just throwing out ideas. I don't know squat about training tracking dogs, but am a little cognizant of things that might throw a dog off.
Killdeer