Prisoners unable to be assigned cells... can't figure out why

Screenshots

I've got unassigned cells for all security categories, and even designated a whole section as shared (used to be an unoccupied min sec) just in case. I don't think it's an issue of deployment zoning either. I have plenty of solitaries free with no one waiting, and this isn't after any intake for a couple of days (as I'm doing the low food part of the Inmate Nutrition Grant). Since the last intake everyone had a cell and I only got this notification after I started an execution schedule then cancelled it. Is it to do with death row? I coincidently have 2 death row inmates... but they're sleeping soundly in their cells.

Any thoughts, help or pointers on the bleeding obvious I haven't seen would be greatly appreciated!