I think it's common amongst school horses as well. They become bored and know their job coupled with students, like spyder said, don't know how to ask for more or don't want to risk waves. There needs to be some spark, even if it's just a sign of enjoyment.
I haven't seen it in Dressage, but local hunters have their fair share of horse (especially the flat classes) just going around, ears floppy, eyes dull, doing the correct but bare minimum.
It's enough to make you want to take the poor horse out for a nice brisk hack.
If your horse says no, you either asked the wrong question or asked the question wrong
And God took a handful of southerly wind, blew His breath over it and created the horse