Update 4/15/15: So I tried adding hay around the ollas with lots of exposed soil and adding a little bit of shade--no joy. I still had to keep adding about the same amount of water to them. Then I had an "aha!" moment. The issue probably isn't exposed soil or shade. The reason established plants probably need so much less water is that their roots have had time to grow right up next to the ollas. For instance, check out this picture of an "inverted root ball" I found on the Internet:
All that light brown stuff is a big mass of roots that grew right up against the olla. Once the olla was pulled out, the roots were formed in a perfect olla shape in the soil.
So the reason young plants need more water than established plants is that they haven't attached themselves directly to the olla yet, which means more water is needed to pass through the soil to the roots. Once the roots get more established, they'll be pressed up right against the olla, and no extra water will be required to pass through the soil. So the extra water requirements for young plants is a purely a temporary situation.
So there you go. Those are the factors that I have found that may impact how often you have to fill up your ollas. In other words "mileage may vary depending on use." I'm still a huge fan and think they are a great way to save water. I'm just always looking for a way to make them more efficient!