I'm glad someone brought this up as I was about to ask myself. I think there are several reasons why you're not getting varying distances with varying clubs. In no particular order of importance:

1. Your swing mechanics is wrong i.e. loss of power, not using clubface loft to lift ball.

2. Clubface is not getting clean contact i.e. not sweet spot contact.

3. In order to generate some power in the swing, you need to have strong muscles, especially in the core areas i.e. spine, abs, shoulders, arms, wrist, hands, thighs, hips, basically overall good fitness level.