I ran into this issue a while ago and from what I remember it was because the physics asset was out of scale compared to the skeletal mesh - the engine thought the body was off screen because it was so big.
I have found that creating a completely new physics asset in PhAT for the skeletal mesh resolves this issue. Just make sure the physic asset bodies resemble your imported bone structure.
There are some great video tutorials on how to set it up if you haven’t already done so.
This is the documentation on it.https://docs.unrealengine.com/latest/INT/Engine/Physics/PhAT/UserGuide/index.html