At a guess, some of the files for the tracks in the track groups are not in the location they were in when you added them to the PlayIt Live library.
I've not seen blank items in the list of items to be played before but I wonder if your clock is scheduling a track group with no valid members.
As a general observation, PlayIt Live likes an absolutely static set of files for the library. Don't point it at a massive library of MP3s. Copy those you want in your rotation to a structured set of directories that match your track groups then systematically add them.
Ensure the metadata (ID3 tags) is clean and the tracks are trimmed and level adjusted as you want them before adding to the library. Also make sure that the in, out and vocal starts points are correct.
During live play you can always drag in another file but a small and well curated library seems to work best.