I noticed something that my friend brought to my attention is that you pull your horses had torwards your knee and your horse will naturally go torwards the pressure then turning circles you just keep holding that position until they stop then you pat them and release. After that you go to the next side and do the same thing.
After doing this for a good couple weeks sometimes a month for the slower horses lol then your horse will flex like a pro. Oh and don't forget to continue the exercises even after your horse perfects it. I did this with Junior a couple times and it helped his knees and flexing all at the same time. My other friend even did it with her Tennessee walker he flexs alot better now and he used to never bend like that. Its really a good trick you should try it.