On the Patron Preferences tab, there are fields in the preference details for Last Sent, and for Last Sent Date. Do you know just what they mean?
Last Sent Date
The Last Sent Date indicates the last time the Nightly Programs used that subject to assign a book, or add a title to the Service Queue. If that date is blank, then it hasn't been used at all yet, or it's an exclusion.
Sometimes, you may see a date from a long time ago. This would likely mean that the subject doesn't have many books associated with it, or on physical circulation, that you just don't have many (if any) copies of books with that subject. You might also take a look at the patron's exclusions--someone who wants Westerns, and has exclusions against Violence, Strong Language, L'Amour, Max Brand, and Johnstone is likely to run out of Westerns fairly quickly.
If it looks like Nightly should have plenty of options to pick from for that subject and it still appears to be getting skipped, you're always welcome to send us the patron ID and we'll help investigate!
Last Sent Yes/No
This field is keeping track of which subject was used last to select a book. You can use it to check where the system is in the rotation through the preferences. The next time the patron comes up for service during the Nightly Processes, the system will start the subject selection with the one AFTER the subject listed as Last Sent=Yes, with the subjects sorted by the Order number (and then by an internal identifier, if you have multiple subjects with the same Order).
I hope this helps answer some of your questions about the Last Sent info!