Hi, I'm completely new to Maya but have been following a tutorial on Lynda, I've got up to a point where I need to start animating the model and have been trying to pose it by rotating down the arm joints (the model is a bug so it has four arms) but for some reason one of the arms refuses to keyframe and whenever I rotate it down, if I go back to an earlier keyframe it has stayed sticking up?

Basically however I rotate the joint (which is a NURBS circle with a rigging attached to the shoulder of the skeleton) it just stays stuck that way even when I select and keyframe it, although all the other joints are working perfectly.

Does anyone know what I may have done wrong during the modelling stage to cause this to happen? Thanks for any help in advance.