Sand dunes are constantly in motion due to weather and land shifts and wave action. These dunes move 1-2 feet a year, I know, doesn't sound like much but look at these pictures. We were there in mid-late August. And the benches you see here were completely uncovered and usable not two weeks before.
This one was the least buried of all the benches we saw. We could only find the boardwalk in one place, way out at the end of the walk and then only for maybe 30-40 feet.