Warner Bros. and the Injustice development team have identified the cause of a matchmaking problem in Injustice: Gods Among Us Ultimate Edition. Reports from the Steam community have been very helpful in pinpointing the problem.
A server configuration error was introduced when the game was made live to the public on Nov-12. We have been in contact with Valve, and Steam engineers are working hard to fix this issue. We will update this community as soon as new information is available.