I've been reading about Peter's strange problems... and as fate would have it, I started to have the same problem...
What worked for previous incantations of the P2 wasn't working on v27, but with a twist. I know what happened.
While working away in v27, an OV7675 camera board at 3.3v from a variable power supply, my camera board developed a short. I know it shorted because I could smell it. It stopped sending images, and the variable power supply hooked up to it showed 0V... that is zero as in zero volts, not OV as in Omnivision:)
I plugged a fresh camera board in and by golly it didn't work either. No smoke or anything, just couldn't seem to get a vertical sync to save its life. I went back to Pv12... and after moving jumpers around, I was back in business. The only thing is... the exact same code won't work with Pv27a, even though it had before. No vertical sync, which remained on exactly the same pin that works for Pv12.
My conclusion is that the fabric which connects to the pins differs between Chips various incantations.
A damaged fabric might affect one version of the P2 but not another.
Case closed. Time for a drink.