Without knowing how far out you're trying rent (ie, pickup on what date), it's hard.
At least at Avis (but I suspect at other places too), the lowest rates are found by making a pay-later (no card charged until you pick up the car) reservation and then start checking two weeks out or so on a near-daily basis, and at many locations the price will drop (sometimes plummet), though after a few more days it may rise (or skyrocket) again.
This is assuming no special event is going on in that area at the time you're going. For example, don't plan on this technique working in whatever town hosting the Superbowl for Superbowl weekend.
In other words, rates vary over time (both the time of the pickup date, and how long a time before pickup you finalize the reservation) way more than they may vary based on anything else.
Try some completely different pickup dates and see if the problem is the specific pickup date you're trying to book.