DHCP config missing default gateway

I have this problem with some computers missing the default gateway, all others settings are correct. I have moved those PCs to a different port/switch and after releasing/renewing the IP sttings some aquire the right info but I can’t point to anything as it happens intermittently.
We have Cisco 3550 multilayer switches connecting all PCs and this school has been moved to it’s own VLAN/SUBNET recently, (we have done this with 10 other schools in the district)and have not seen anything like this.
Any suggestions?
Thank you in advance
JOGO