Domain not available

I have a school network with 2 domains, both running win2003 server and connected via cisco router and Cisco switches. Two VLANs have been set up on the switches and ports assigned to respective VLAN. My question is; some of the PCs in VLAN 1 will log on to both domains (ie student can logon to VLAN 1 and teacher can log on to VLAN 2) with the PC connected to a port assigned to VLAN 1, but 2 of the computers will only allow log on to VLAN 1 by a student. Teacher cannot log on to VLAN 2 from that PC – error message says domain is not available. Any ideas on what I can do to rectify this?