Since it is located in a shed it isn't in a controlled environment like inside a house. Humidity and such play large factors. I would recommend an enclosure like this: https://www.homedepot.com/p/Greenfield-Weatherproof-Electrical-Box-Lever-Switch-Cover-with-Single-Pole-Switch-Gray-KDL1P/202188640
I've installed them before, they're very easy, and your switches should last longer since they'll be more protected. Also, check and sure nothing has been leaking in/on the switch or electrical box.