Have another go at straightening it, over-bend it a little so when it relaxes it relaxes to being straight. Straightening some woods has to happen over a series of sessions, sometimes you might have to tweak it 5 or 6 times. Some staves just refuse to stay straight no matter what you do. Cant remember if you used steam or dry heat, but whichever you used last time, try using the other - you often need to use dry heat to make a steam bend permanent. Also, you can both straighten and temper bow at the same time with dry heat