Restricted thumb movement in mecanim?

I have a project that's all about hand animation. I build my anims in 3ds max, same with the hands themselves.

Mecanim works well for this in the main, but I've come across a huge problem. When I pay close attention to the animations playing in the editor I realise the thumbs aren't in the right place. They should be pointing down more, or further to the left etc.

I exported another basic test animation where the thumb bone (proximal) moves through each axis one at a time. Turns out only one axis is coming through.

This is a massive problem as thumb placement is key in a couple of anims.

The thumb test I exported on it's own and set up as it's own independent rig so it shouldn't be anything to do with interference from sticking anims from one biped into the rig of the actual on-screen biped.

Any ideas?

I checked around, found someone with the sam eproblem but he solved it by having his bipeds have identical naming conventions. I haven't changed my biped naming ever so it doesn't apply.

Any advice welcome.