A horse question regarding...?

Brief：
Wondering if you can help me out. My horse Meeko is a nine year old paint. Ive had him for exactly one year tomorrow (very happy owner). I have on pro.. 

Wondering if you can help me out. My horse Meeko is a nine year old paint. I've had him for exactly one year tomorrow (very happy owner). I have on problem with him- he doesn't respond when I try to make him trot from the walk.

He just keeps walking, no matter what cues I give him. I use my seat, my leg, and now even a crop (which I hate). He'll go for a time, then go right back to the walk. He trots fine on the lounge with just a vocal cue.

He'll canter fine from the trot if I do it quick enough before he breaks back to the walk. It is very frusterating, and I am tired of fighting him whenever I want to ride.

What should I do to lighten him up?

First off it sounds like something may be hurting him at the trot, but since it is only when you are riding him and not on the longe then you should check his tack. If it is his tack then keep in mind that the trot is a bouncy gait and it might not hurt him so much at the canter.

It could also be that since usually in a work out you would walk the trot and then canter so he just hasn't woken up yet and is taking a "Sunday morning stroll" and then by the time you canter an have been trotting for a while then he's already awake from the trotting. If this is the case then keep him at a brisk walk when you warm up-- don't let him just drag his feet!

It could also be that he has a back problem and once again, the trot is a bouncy gait so it may hurt him more than a canter. Are you sitting the trot or posting? Have him checked by a chiropractor.