Ensure the tracker offset for elbows is slightly positioned away form the mannequin, in this video you can see how the elbow tracker offsets are about a foot or so away from the mannequin's actual elbow!
Elbow trackers are supported but sometimes require slight adjustment to get exactly right. Check out the video that shows how to manually adjust the elbow trackers from the TPose mannequin.
Make characters even more animated using the built-in mocap editor and VR tools.
See Also:
Important tips when using SteamVR Lighthouse optical tracking.