I was able to resolve all problems I had with my library. I only have just under 3000 songs but I ran into all of the same problems discussed in these threads. The first thing to do is to right click the column headers and add the iCloud Status column. This will tell you why the little cloud with the line through it is showing for a particular song. I found two different reasons for all my ineligible songs:
1) it was a duplicate of another song, in this case the iCloud Status will say Duplicate (therefore you can just delete it because it has already been matched or uploaded). If you are concerned that this isn't true then just go to the album in question or search for the specific song and you will see two versions.
2) It was encoded in something other than what iTunes will accept. In these cases I just right clicked on the song and selected Create AAC Version. Once the new version is created, right click it and select Add to iCloud. You can then delete the other version once it is uploaded.
With regards to the Music Video problem many of us are having, iTunes will not upload or match any music videos that were not purchased from iTunes. This is an obvious problem and oversight since there is no other way to sync them once you enable iTunes Matching. My short term fix (hopefully will be short term anyway) was to select all the music videos that were not uploaded to iTunes, right click and select Get Info, select the Options tab, then change the Media Type to TV Show. This puts them all in my TV Shows section of iTunes and I can sync them with my iPad and iPhone using this. Not perfect, but it is a usable workaround.
Hope this helps some of you, ciao!
Steve