Could be boredom, to not wanting to pick up his feet. I say switch things up. And then come back to that.
Thanks I will try your suggestions. Yes boredom could be a problem as she's very forward when we ride out.
I don't believe in putting spurs on or stronger bits or anything. I ride with a dressage whip just because it is handy to have, I know the strength that I put into it every single time and I have full control of it and can drop it if necessary. Spurs aren't so forgiving.
He may be a little bored? Try switching things up a bit like polework or activities. Even changing gaits and directions often would help. Otherwise try doing your schooling on trail rides and then revisit the arena after a while again when he gets tired of the trail (it's usually the clever ones that are like this). I have a Warmblood that gets sooo bored that I can't even get through a warm up. He just wants to outride the whole time.
If it isn't a boredom problem, try lunging him in the arena and get him moving off of the cues easily and swiftly. Ask once, tell, then make him move. My Warmblood also used to be SUPER lazy on the lunge that I had to run next to him to get him to canter. Now all I do is stand in the middle and use a verbal cue and off he goes. It makes for a much more pleasant experience for you and quite frankly your horse too.
If it isn't any of these problems, maybe he just isn't fit for the arena work or he is sore somewhere. Get your saddle fitter and/or vet out if you feel like his strides have become more shorter or chopper or if he bucks or something like that.