Advice on some fundamentals and behaviours

I'm still new to flyfishing. My casting is going well and I'm catching fish on dry and wet fly, which is a real pleasure.

I'm puzzled at the way I'm catching fish on wet fly though and I'd appreciate some experienced insight.

I should probably first describe the fishing I'm doing and the fish I'm catching... I'm fishing river for wild brown trout. So far I'm catching small fish no more than 7" in length. I live in hilly country (South mid-Wales) though most of the stretches are quite broad, running through wide, flat river valley flood plain. The river bottom is pebble, and in places sandy. Flow is brisk on the runs, especially in flood, and slow or steady in the pools. Depth on the runs is ~ knee deep, sometimes more or less. Pools can be deep, 6 to 12 feet.

Back to the fishing. The only time I catch on wet fly is when I'm retrieving at speed. I have never yet caught by casting up stream (and taking up the slack as the fly returns), or casting downstream and paying out line. With trout this size though, it's quite possible I'm getting takes but never see them? Would you expect to fish on the retrieve, or should I be trying something else?

Thanks for your thoughts
Mark