Yes, I had this problem, spoke to 3 separate Virgin customer agents and 1 Delta agent and none were able to help, they gave me a different excuse/reason each time. The fourth Virgin agent was excellent and she was able to assign my seat over the phone, after which time the seatmap opened up for me and worked properly.

I forget the exact reason she gave (something technical to do with how the flight was ticketed) but I get the feeling that the issue is something less experienced customer agents wouldn't recognise