The problem for you is possibly that you expect the horse's head to bob up and down when he's lame? But that's just not how it always looks. A horse can be severely lame without bobbing his head. And the more legs he's lame in, the more difficult it gets to spot. Compare the 'flight path' of each hind leg in motion. Look at one hind leg, then the other, compare their length of stride, how the hock flexes, etc. Also look at this horse's behavior. He's extremely resistant and moves very badly under saddle. He is, in a word, 'unhappy'. I saw a video of the girl trying to get him to canter, watch that, the behavior is very, very typical of a horse that's 'tight behind' (lame in the hind legs). Head lifted, neck hollowed out(not rounded on the top line of the neck), slows down, refuses to pick up the canter. You see, to pick up the canter with a rider when circling, the hind leg on the outside has to push harder off the ground, and the hind leg on the inside has to 'engage' further under the horse's body and bend more. A horse that is unsound in both hind legs will resist picking up the canter in both directions. A horse can 'run down into' the canter, and many do when green, without much change in either hind leg, just by sort of shuffling the hind feet a little, but they have to be sound enough to do that, too. When a horse is lame in more than one leg his gait starts to be what's called 'pottery.' British term, it means the overall stride is restricted and uneven. Try this. Hold a piece of paper up to the video so you can't see anything above the horse's stifles. Part of the problem is that you have to kinda have a picture in your mind of exactly what a 'normal stride' is for this type of horse. You have to have that in mind and compare that to what you see. Look at how the horse is swinging his hind leg in toward the midline of the body while the hind foot is coming forward, in the air, rather than putting it down in the usual spot. It's so severe that the stifle joint is actually twisted inward with each stride. Look at overall, how restricted the stride is. Look at how the foot is put down on the ground. In some judging situations, some judges were trained very strictly to look only at the hooves, then only from the fetlocks down, then only from the knees/hocks down, etc. It helps a lot if you can do that. It takes a lot of concentration. Isolating each part and looking at how it moves IS useful, but it's also really important to look at the entire stride overall, an to how the body is used with each stride. Usually with hind leg lameness the horse's rump will bump up and down unevenly, in other words the hip will pop up in the air on one side when one foot moves. This leads a great many people to insist the horse's back/hip/sacroiliac are the problem. But in general this irregular bobbing up and down of the croup is due to hock lameness. The back, croup, stifle and hip will all be sore as well.