updated 12:41 pm EDT, Mon June 16, 2014
Service timing out for some users
A number of iTunes Match users are experiencing problems matching and uploading tracks, according to complaints on Apple's support forums. Affected users will see iTunes say "Matching your music with songs from iTunes Store," but have the process stall out as it appears to be on the verge of completion. Difficulties started on or around June 9 and are continuing.
The issue is believed to be on Apple's end, since no step taken locally seems to be able to force successful matching. At the same time, though, Apple hasn't acknowledged the situation, and its System Status tracker shows all iTunes services as functional.
One theory is that Match is having trouble coping with music that isn't hosted in the iTunes Store, since tracks that can be found there seem to be dealt with normally. The service should, however, allow uploads of any tracks found in an iTunes library.