My dog loved playing ball... why did she stop?

She's 5 years old, a westie-poodle mix.

She LOVED playing fetch with a tennis ball ever since she could squeeze it in her mouth.

Now- we toss the ball, and she'll half-heartedly run after it and then stop to sniff, or wander off.

Is it a loss in energy? getting older? Something else? She has no other present signs of illness or pain-- though when she was a puppy, we had surgeries performed on her hind legs for luxating patellas, which we're told have healed well, and she has been extremely active since. So, we would make ties between that surgery and this, but she shows no other difficulty running, walking, or jumping.

Any ideas? We're stumped!

My dog loved playing ball... why did she stop?opera ticket

Hmm... I'd take her to the vet to rule out a health problem. Dogs usually don't do that, in my experience.

My dog loved playing ball... why did she stop?pacific theater opera theater

She might be losing energy and losing interest in playing with the ball, try a new toy or game and if she goes back to as she was then she's fine, if not try a vet, she may just be getting older and lazier.
any change in events happened around her? did someone she likes leave the place or soemthing? that could be one reason . other reason is that she might want a mate and she is sniffing one that was in the area . hope she gets back to her real self
well she is probably tired of the game. just like when we get bored of something. she could also not have enough energy since the surgery. she could also not want to play that game because she wants to do something else. she may be getting older and is weak, so chasing after a ball hurts or makes her get tired easily. also the surgery could not have gone well, but she is not showing any signs.