I use treats AND toys the same way. Best to have both rewards in my bag of tricks. Tiny treats (real treats, real small, HUNGRY dog) can be rewarded fast, swallowed fast, and training can immediately continue so dog is still focused and learning.
I'm all about the toy because of the excitement and drive it can add for the training. But a rousing game of tug as a reward is a HUGE break in the training. As is throwing a ball out and away from me (and what we were working on).
So I use the treats (clicker too
100% of the time when I've seen treats not work:
- Dog isn't hungry (skip meal before training, 2 meal if that works better) you should be using tons of rewards so your dog isn't starving, and you feed normal meal AFTER the session
- Handler uses low value treats (to the dog). Doesn't matter if I pay $20 for some great dog treat, if my dog isn't interested it's low value TO THEM. Generally my fridge and my food is WAY higher value to my dogs than anything in the dog food aisle/pet store. Hotdogs, sharp stinky cheese, liverwurst, MEAT and MEAT, tortellinis, pizza.....
- Handers do not treat frequently enough. Using treats is FEEDBACK to our pups that they are brilliant! And if instead of using the treats constantly (in the beginning) so our pups learn to engage and keep trying cause a treat will come in a sec, we only reward a FINAL behavior every 3 or 4 minutes when WE think they FINALLY got it right. Heck, you rather get a paycheck every week or every other month!