I have a horse I would love to adopt with this same issue. Lovely, Lovely ground manners, actually looks deep in your eyes and sighs! Then, if you ride her, the ears go back, and if she moves, its with the good chance of a buck or two. She is very large morgan/draft cross..and so has some power behind those bucks. My trainer will not agree to let me adopt her because she is not trustworthy (and I am 56!)...Are there just some horses that need to be appreciated on the ground only?
I think I would like to clarify what "good ground manners" should mean. It doesn't just mean that the horse stands next to you and looks sweetly in your eyes. It means that whatever you ask of the horse, they do it with a focus on you and a respect, and willingness and promptness. It means they have an awareness of your space and keept the boundary.
So, I would take that a step further. If in the round pen, you ask the horse to go forward, and it doesnt' go forward promptly and at the speed you are really asking (not thinking you are asking but actually saying with your body), then it doesnt have all that great ground manners.
To get the hrose willing to go forward under saddle, you need to have it willing to go forward in the round pen, or on the lunge line. Go there and get your horse to really move out when you ask , how fast you ask and as long as you ask.
I do , however, agree that an unwillingness to move out under saddle is hallmark for a saddle that is ouchy.