Xsi to unity animation problems

Hi there

For my WIP I am modelling and animating a crossbow to be used as a weapon.

Mostly all of the animation works fine, the moving, the sprinting and the reloading.

However when the weapon is fired, the arrow shoots as normal, but the Crossbow string is not animated.

The crossbow string in XSI is animated using a null. All the animations are made using null objects. However the crossbow string is the only animated object that has POLYGONS being animated.

So the problem is, these polygons won't translate foward by animation from a null when exported to unity.

Is there anyway around this? Another method for animating polygons or appointing special properties to the null?

Thanks.