Due to a very unfortunate and at the moment unavoidable problem in matchmaking there is a certain case where your code will not be matched.
If very shortly after your submission another person submits, your chance of getting matched is so small that you can assume it to be 0. If on the other hand the next submission comes a long time after your submit, you have a very high chance to get matched.
In very popular queues the chance that someone else submits directly after you is very high.
The only known solution to this at the moment is to resubmit your code and hope it works better this time. The developers of CodeCombat are working on the problem, but it is not completely solved yet.
EDIT: Found the topic (especially Nicks post explains the current system).