ISSUE RESOLVED!
Umberto, you probably would not have been able to duplicate the error unless I had sent you a copy of the modified AFCAD. I moved the aircraft to the right to the next stand and all works perfectly. I think the problem was that I have placed the position of GATE G2 too close to the left edge of the tarmac and there was not room enough on the left for the bus. I will either remove the bus call for that stand or move its footprint to the right in the AFCAD. Anyway I have resolved the issue.
Craig