Maybe he doesn't like you. Compliment him a bit. Make small talks. He was imprisoned and tortured for a long time he need a sign from you.

But seriously 99% of such quest bugs can be fixed by relogging. I don't know if you fix this one but keep that in mind for future refference.

It's been a long time, but I seem to remember you had to wait a few days game time between halls of torment and valley of the blades. Seems Sai needs a few days to recover. Go do something else for a little bit and the Prophet will show up and tell you to go see Sai.