Why does my horse buck?

My horse is a 8year old warmblood but when you ask for canter he gets faster in trot and then bucks. Although when a horse is next to him and canters first he will canter straight away. Its just when you put him infront or canter on your own he will buck when asked for canter. He also bucks when asking for canter on the lunge rein and then just canters. We have had hes back checked and it was fine.