This is frustrating and it happens to me too. I've been very methodical about iMatching my library. Here are some things to try.
1. If you haven't already started, move your music files to another location. Start with an empty itunes library. Add songs (drag to itunes) one album at a time. Get that album matched, delete the local copy (keep iCloud version), then move to the next. It's easier to tackle an album's worth of songs at a time than thousands at once.
2. After adding an album, highlight the songs, right click, add to iCloud.
3. If any songs do not match (status bars says 'uploading...'), stop the upload. Right click those unmatched songs and 'create MP3'. Then select the newly-created MP3s and add to iCloud. For some reason, this works alot of the time.
4. If a song won't match, check the same song's length in the store. I've had many CD rips where a song was a couple of seconds longer than the store version and it wouldn't match. Get Info, options, adjust the end time to match the store's length, then create an MP3 version of the adjusted file. Add to iCloud. This will only work for songs that too long, not too short.
5. If all else fails, re-rip your CD and try again.
I still have several random songs, ripped from official CDs and encoded by iTunes, that refuse to match. But I have most of my 5K library matched.
daboneyard's tips are great advice, I managed to match several of my uploaded tracks by adjusting the time, re-ripping or converting them. What worked best for me was recording the tracks: I used Audio Hijack Pro to record the Audio and create a completely new file. That way it is also possible to add a couple of seconds (to get the same length as the iTunes Store-version). Sometimes it wouldn't work at all and sometimes it would only work if I recorded the exact same track off of Spotify - but it did work most of the time (doesn't make any sense, I know).
Anyway, it is very time consuming to do that... but for me it was worth it, I got about 70% of my formerly uploaded tracks to match.
sigh... i contacted itunes store support to see if they had any suggestions.
all i got was standard "copy and paste" responses that weren't really relevant to the issue.
then after explaing it again, their solution was to offer me 5 song credits in the store to download the tracks!
now whilst i appreciate the gesture, it doesn't really do anything to solve (or even attempt to investigate) the issue.
I thought about contacting iTunes support but figured they would have no idea how to solve the matching-issues either. It's completely arbitrary, god knows what's going on there. Most of my mis-matches just don't make any sense at all and it's usually just one or two songs out of an (otherwise matched) album. I am not exactly hopeful there will be a solution any time soon... allowing manual matching would be great, but I guess that's not gonna happen - I can't see the record companies loving that idea
Anyway, I already did everything I could think of to re-match as many tracks as possible, I'm gonna leave it at that and wait if Apple comes up with anything.
I end up buying TuneUp companion (http://www.tuneupmedia.com). It's matched most of my songs , even the ones with Cyrillic or Hebrew names, downloaded the album art, and updated all the info in the iTunes.