I have enabled Sync Between Computers using the same Ubuntu account on three different computers.
When I open Sync Between Computers in Software Center I only see the current computer. I used to be able to see the two other machines and could access a list of the packages I had installed on the other systems. I have the same problem on all three computers.
How can I resolve this problem?
Software Center issues tend to need a reinstall to be resolved. The best thing to do would be to deal with it until the 17th, when Ubuntu 13.10 comes out. Then, you will want to reinstall anyway. That should fix your problem.