Mark is having trouble getting iTunes Match to replace a lot of his music with higher quality AAC versions. Mark says that he's gotten 70% of his music matched. Leo says 70% isn't bad.
The first thing he should do is make sure those files are higher than 96kb. Then, the music needs to have the songs in it's library. CDs he rips may not be in the iTunes store, and if they aren't, iTunes can't match them. MacWorld has a great guide to using iTunes Match.
Another thing to do is backup all of his music and then delete the music that has been matched and has copy protection. Then run iTunes Match again.