Sounds like they just don't wanna. If there's no pain (and you're sure of this) I think you need to make forward movement more fun. Cantering around and arena is really boring - they're probably burnt out from doing so much of the same old stuff. If it's safe to do so I'd try some trail riding, find a good area to canter they should be more eager then. Riding should be fun for you and the horse, not fun for you and a chore for the horse.
O'course there could be a number of reasons they're stopping, heavy hands, discomfort, not being used to the bit. Contrary to popular belief single jointed snaffles aren't that soft. Single jointed snaffles with hard hands are a bad combination. If you really want a snaffle not a leverage bit try a french link. If they're used to leverage bits a kimberwick might be nice, if you put the reins on the regular ring, not the lower hole, it's a fairly soft bit. IMO softer than a single jointed snaffle, harder than a double jointed one.
Could be pain related problems of course, could be they're stiff - they could just be out of shape. If you haven't worked out or done much physical exercise in a few weeks/months could you get up and run laps without being winded or sore?
Could be lots of things but I'd check your bit, keep em fit, and keep their minds interested in what they're doing.