iTunes writes to an XML file in your music folder, which contains references to all the files in your library. This XML file is utilized to access your iTunes library via Scratch Live and ITCH.

In cases where you have tracks with characters that iTunes does not deal with correctly iTunes will stop writing new tracks to this XML file so Scratch Live, ITCH or any other program that uses this file will not have access to new tracks you've added.

If new music you are adding to iTunes is not showing up in Scratch Live or ITCH then one thing to check is that your iTunes XML file is being updated correctly. 

Your iTunes Music Library.xml file is kept in the iTunes folder of your Music folder. 

If the date that this file was last modified is older than when you last added music to iTunes then iTunes is not correctly updating your XML file.

The best thing to do to remedy this is to open iTunes, sort your library by Date Added and look at the files that were added around the last time this file was modified.  If the tags or file names of these files contain accented or non-English characters they will be preventing iTunes from writing to your XML file.  By changing the names or tags of these files you should be able to get iTunes to write to your XML file again, and recognize these tracks in Scratch Live and/or ITCH.