Webaccess

I have 5 Netware 5.1 servers containing POA and one of the 5 also has the MTA.
Everyone is able to log into their PO using Webaccess, except those whose account reside on the server containing the MTA. The error is “post office not available”. It would seem as though there is aproblem with the POA on this server. In fact, my workaround is to move mobile users to one of the other servers. Any help please?