It's really hard to see, but he looks a bit too far away from the cart. You need to take your traces up in your breeching too, so he doesn't step over it.
It looks like your breeching has slipped alarmingly too, so something's going on there by the looks. It shouldn't have done that.
Are there no terrets on your collar, your reins are in danger of getting caught up. Looks like there is a ring on the neck strap that supports the collar.
What we really need is a photo of him standing in the cart, all done up in his harness on flat ground. Just looking again, you might have the belly band far too tight which is not allowing the shafts to float.
Sorry, there are so many factors that could be giving you a bumpy ride. Tricky thing is, if you make one strap longer you will need to balance it out somewhere else, making another one shorter.
Here, can you see what I mean about taking your trace up with the breeching strap when you do it up around the shaft?
Getting harness adjusted just right is a very tricky art.