Not sure why this is happening, but look at the image posted here. The mesh moves with the bone on the arm, but rather than fully moving with it the bone comes out after a certain amount of rotation. Any idea what im doing wrong?
Put the mesh in Weight Paint mode and check to see if that area of the arm has weight from the correct bones. It may have not enough influence from the arm bone, or it may have influence from another bone.
im pretty sure it has all the weight around that part of the mesh for that bone
don’t just be pretty sure, go and check! and make sure you check that none of your other bones are affecting the mesh
ah i didn’t think to check the other bones 
we trust that that means you got it working?