If your engine starts backwards when you hand flip forwrds, then try hand flipping the prop backwards, and the engine will start forwards.

Or flip a lot harder to drive the piston past TDC before it fires. (not what I would do though)

As it was pointed out, the flip is causing compression and detonation before the piston reaches the top of the stroke. This is also sometimes a sign that you might be pre-detonating when running. You might try lowering the nitro %, or installing a head gasket, or going to a colder plug so that ignition happens slightly later. Though I honestly think all of these "fixes" are overkill, just try flipping the engine backwards to make it go correctly.