We have two cottages side by side. One is fully booked except one week in the summer (and was booked probably 3/4 of the spring). The other, though it's very similar, is about half full for the summer and was probably 1/2 booked this spring. It always books a bit slower than the other, but usually gets booked. It's interesting because the cottages are very similar except for the decor/colors. So, I'm assuming I need to brighten up the colors in the cottage that rents more slowly because the brighter colors show up more on the VRBO photos. I, too, get a bit nervous this time of year if the cottages aren't fully booked yet.
I think some vacationers feel there are a lot of places available so they feel like they can book closer to the season. In part, that's certainly true with the building that has been going on. The only risk with that is that they'll have less choice in terms of size, cost, location. But for many people that doesn't matter because they're at the beach and any place (within reason) is nice at the beach. But if it rains and people have to stay inside for a while, it's great to have a nice place.
Good luck on your rentals, CTJake.