So you are subscribed to iTunes Match? If your songs are only stored in iCloud, and not also stored locally on your computer's drive, you cannot sync your iPod to those songs. Before iCloud, ALL of your songs were store locally on your computer's drive. That's why it worked when syncing your iPod. Did you delete the locally stored song files after you subscribed to iTunes Match?
You can download any song that is in iCloud so that it is stored locally on your computer's drive. In your ITunes library, show your songs as a list. Right-click in the header area of the list, and select iCloud Download from the list of list column choices, if it does not currenty have a checkmark. The columns currently shown on the list have a checkmark. On the iTunes music library list, the iCloud Download column has a "cloud" button for songs that are in iCloud but not stored locally on the computer's drive. Click that button to download that song. You can also select multiple songs at the same time, right-click on the selection, and select Download.
Once the songs you want on the iPod are downloaded and stored locally on your computer's drive, they will sync to the iPod.