Two things I can think of.
You're too ducked – knees forced too wide, weight onto outside of feet instead of on the balls
Your boots might be too narrow. Most brands have different fits. I was recommended Burton when I started and I stuck with them. Bindings to match the boots, Burton-Burton, Northwave-Drake, etc.
Edit – Try standing on the board in sock soles, no bindings. Squat down with your stance at the right width and see what sort of angles you're looking at, with your knees feeling strong as possible. I used a lot of duck when I learnt switch but when I took it off I found I was a lot stronger over the board.