Scorn's tail-arm comes packaged in a configuration that's not conducive to forming the tail properly. If you look inside the tail's groove you can see a grey plastic tab. Wedge your fingers in there and grab it with one hand, then hold onto the tail with another... then you gotta wriggle that damn sucker until the tail is set into this position:

Once that's done, the tail will sit flush with the dino mode's rump. I never bother resetting it back to the factory default position; as I find this position doesn't impede the robot anyway.