It sounds like the both of you are frustrated now. Could be you are expecting too much and not breaking it up into small enough pieces.
I think John Lyons says that if you horse doesn't do what you ask and it's 'cause he doesn't understand, you need to back up to the place where her does understand and go just to there, no further, then little by little work to the next place .
So having your horse (name?) flex from side to side, just a bit, from the halter would be rock bottom preperation for getting good bend. Do that until that is really successful. Then flex more and then flex and step under with hind.
Once you have that on the ground, bridle up and do it under saddle.
Go slowly, mix it up and do some nice old canters the way HE likes, too.
If he cannot bend due to physical stiffness, then you have to get there slowly. His body has to change a bit, and that takes time.
Always give plenty of rewards and breaks.
Full body bending is almost an illusion. The horse cannot really bend his body very much , except in the neck area. He can, however, tuck his pelvis under (like you may see him do whe he is pooing). Doing this allows him to reach his leg further under his body and even cross toward the very middle or infront of the outside hind leg when going on a curve. THIS is what we feel and what makes the barrel of the horse swing to the outside. Feels like horse has a lot of bend (and he has some) but it's the stepping under and across toward the outer curve that we feel as bend. THIS is what engagement means.
That will be what you want to see happening, but don't expect it tomorrow. Start small and end on success. Have fun and don't practice the same thing every day.
Take a video sometime. Wish I was there to hang out and play horses with you and your guys.