you must be sure that the MTU is the same on all the equipments including servers, the default is 1500. On switches for example it is not shown with the show running command by default unless someone have changed it
I got some issues with MTU, I have to be sure of the MTU configured on all switches , routers and firewalls (I left the default MTU parameters) were the same and I configured the ip virtual-reassembly interface command on my router, it works to me
See this link, it is about ip virtual reassambly concepts and how to configure it
http://www.cisco.com/en/US/docs/ios/12_4/secure/configuration/guide/h_vfrag.html