It's very possible that they are all taken for upcoming reservations. I do see that Avis, Budget and National all have availability (we can discount them pretty well).

There is no way to force availability online that I know of with Hertz. I would call the location directly at 309-663-5201 to inquire. They may be holding them back for whatever reason and can force it through via their counter system for you, or at least explain why you're not seeing avail. online.