Hi,
They do skip some years, and this ften seems unrelated to weather, etc (at least to us!). However, these and other frogs are also very sensitive to pesticides, changes in pond pH brought on by acid rain, etc. Breeding populations also will relocate if a more favorable habitat appears. Grey treefrogs are especially mobile, and good at finding new places to breed...I've seen them show up at a temporary construction pit that filled with water - at the edge of a busy parking lot in a NYC suburb; they also breed within NYC. Hopefully yours have found a good spot, or will return when conditions cghange. Best, Frank