DHCP/Switches/Terminals/Citrix

Since upgrading our hubs to Cisco catalyst 3500series switches, we randomly experience our terminals not being able to receive an IP address from the dhcp server. Clients will have to try twice or three times to obtain an IP so they can logon. This did not happen with the old 3Com hubs that we replaced. The duplex on each side of the switch matches at 10/half which is what these legacy terminals (WYSE 2300 series) operate at.
any ideas?