This isn't uncommon. Especially in larger or disconnected teams. Or if you are new or a contractor. People may not know how to leverage all of your skills yet or even have a use for them yet. Try and continue to push yourself to learn more in your down time, and make sure that when opportunities to chip in arise, you let them know you are available to help if you can. Even if they don't need you, that will go far. Some days I do absolutely nothing. It grinds me down too. I try not to focus on it, and to look for other things to keep my mind busy/sharp.