3010cn drum/transfer belt problem.

I have bought a 3010cn to print a society newsletter, but itv wont print and says that the drum need re-seating.   No good, so I looked up the error codes. Research on forums showed me that the transfer belt - I think that is what it is - which is a wide rubber  belt next to the drum was not centred.  As instructed on the forum I managed to move it back to centred, so the little shiny square was aligned with the two ridges on the left.  But, after a couple of prints it goes back to off-centre again, and wont print.  What can I do to make this excellent printer work reliably?

Thanks in advance!

Chris