Well, if you always had 2 sockets and your license was for one socket in 5.5, it was maybe something "weird" with your license.
Just check your license in the console if it supports one or more sockets.
If you don't have it, you will have to purchase a new one with the correct sockets.
As far as I know, you need a new license from 5.5 to 6.5: VMware Knowledge Base