Certain animations in inventory don't show on pull-down list when trying to add to gestures. Why ?

[Lancelot Skytower]

I have an old gesture that used to work, and still does mostly, but the animation won't play. The animation works fine when i execute it on it's own from my inventory.  When I open the gesture to try and modify it (or try to create a new gesture), the animation does not appear in the pull-down list of choices.  Anyone know why ?

Thanks