+1................... repetion is a good thing but in small doses,better to chuck the bumpers once and have it be a snappy out and back retrieve,quit on a good note and then repeat the same drill the next day or eight hours later .You do NOT need to run the dog until it fails so that a correction is needed............... that's not training that is OVER training with a negative ending.When you get to that stage,the dog will get flustered and continue it's downward spiral until NOTHING is right and THAT"S when they nuke down and won't play funny little reindeer games anymore.It's been proven that a dog has about a twenty minute attention span,thus that's about how long it should take to get a specific owner desire ingrained.................. on multiple days of course,start small and continually build on the big picture.
bored. Put it away for a week. Take him out, throw two bumpers, put him away.
Find a different way to exercise him.