Have you tried using
tuggng on tuggy-toys, instead of fetching toys?
My dog Buddy seems to much enjoy using tugging on toys, as his reward after a click,
especially if it is a trick he LOVES, like "pick up toys" trick.
does your dog like toys in general??
My dog goes crazy-gagga-nutz for brand new toys, maybe if you got a brand new tuggy-toy, and use that one ONLY for clicker-rewards? Like the dog never ever gets access to that toy, except when he is in training.
(my particular dog would probably end up focusing TOO much on the new toy, and not do his tricks well, though
)