After trying various fly fishing tactics, I'm totally hooked on dry fly fishing. I've had 2 reasonable days this week fishing dry's, mainly klinkhamer's. My main downfall though is that I witnessed many trout come up to my fly, only to turn away at the last minute. My gut instinct was that it may have had something to do with the leader being visible to the fish. I've been using Wychwood Truefly Finesse 4lb / 0.148mm. The leader was floating on the surface at the time, should I be trying to sink it or leave it as it is. Help & advice would be much appreciated along with recommendations on quality leader material for fishing dry's.

Thanks!!!
PS I've only been fly fishing for 2 months, have LOTS to learn