Bones wont import back into ue4

My gun is already rigged with all the bones needed. I exported the gun skeletal mesh and then animated the bones needed inside of my 3d program then when i import it back into ue4 (the animation only not including the mesh) I get this warning show up.
When i preview the animation in ue4 the animation looks fine its all working but when i add it to my gun it doesn’t play but if i use another animation on it, the other animation will play fine. So it must be something to do with the warning and im not sure how i can fix it.