Hi there,
Yesterday i spend all day trying to import a range of animation within a single file. It just didnt work until i realize this:
4.1 uses FBX 2013
4.2 FBX 2014
Im not quite sure if 4.3 uses FBX2015 but if you brought unity assets or from some third party you need to tell them to use FBX 2014(some cases) to export the animations(at least that was my issue)
I hope some moderator or staff read this and update de ue4 documentation.
Let me know if this was usefull.