Some skins don't appear in drop-down list.

I downloaded about six new skins to test, but some don't show up in the drop-down list. I've made sure I have the updated program. I've rebooted numerous times. I've made sure they are in the correct folder (plugins | weather | skins). The folder clearly lists nine skins, but only seven show up (I'm missing Simple White and Weather 16--does anyone know if there are problems with those?)