MixingTransform causes jerky blending

Hey, so I've got a model blending between several poses (since this is for a driving game, at the moment those are just "turn left" and "turn right"), but I have a slight issue that seems really odd. The two turning animations blend very nicely (they are just static poses, and all the actual animation is blending between them), but I want the character to turn around and look behind them when they reverse. Of course, I still want to use the turning animations while this is happening, so I used AddMixingTransform to the reverse animation on the neck bone, and when I reverse, it smoothly transitions into the reversing pose no problem. However, when I attempt to turn while reversing, all blending has vanished from the turning animations, and they just jerkily skip into each other. I've tried setting different blend modes, and adding MixingTransforms to the other animations, but nothing seems to work (in fact, setting the blend mode to Additive stopped all animations working). Does anyone know what might be causing this? I'm quite keen to find a solution...