Did the whip work? If so, I don't see why you don't just use that.
Have you heard of "the lesson of the leg"? It's a remedial exercise for a horse that likes to ignore leg aids. I would only do it in an enclosed arena, and also you do have to have a reasonably good seat before you try it. You will need a whip. What you do is, first apply your usual leg aid for an upward transition. The horse should move out promptly and with noticeable energy. Not just a sluggish amble. If that doesn't happen, you then apply a stronger leg aid. Both times, you are applying the aid only one time--no nagging. If the horse doesn't move forward smartly and promptly from this, you then (make sure your reins are loose and) apply the original soft leg aid and smack with the whip at the same time! You should be smacking hard enough so that the horse jerks forward. The reins need to be loose so that you can guarantee you're not yanking on his mouth when he moves forward. If he goes faster than you wanted, you let him go at that pace for a minute or so, praising him, and then quietly bring him down to the pace you want. You'll probably have to do this a couple times, and some horses need reminders every few rides, but it really works.
Because the horse may go into a faster gait than you expect, you may prefer to get into jumping position and have your hands on the mane for the whip smack, rather than just loosening the rein.
By the time you read this I guess you'll have had your ride, so let us know how it went!