Each plane requires it once per year. Often times, checks can get lumped so many of them happen at the same time, so that might be what you're seeing.
Taking a quick look at your airline, there's a couple things you might want to consider doing to improve things -
1. Your load factors are sky high because you're underserving demand. On routes where you're supplying half the demand on the route planning page and getting full loads, boost your prices by 20% to start, and if it stays full, keep adding a few percent until you start losing pax (give a few game weeks to normalize after each adjustment).
2. Your props are flying too far - not on the extreme side, but they're still going to take 2x or more time to fly some of your longer routes than jets, which will basically only allow them to make 2 flights per day. After paying for leasing and staff, you really can't be very profitable. If you can shorten the leash on them and fly them under 500nm per flight (or even 300nm per flight), and then look into jets for longer flights, it should improve your position. I wouldn't add another fleet type at this time though.