fish2much, I have found that often missed strikes have more to do with the presentation and the reaction time of the angler, rather than you the type of fly itself. One tip for lake fishing is to try using a shorter amount of fly line and a stiffer leader. And as you mentioned that you are using a sinking line, I'd suggest that you consider using fluorocarbon tippet material as it tends to possess less stretch than monofilament and thus enables a quicker and more efficient hook-set when a fish takes or strikes at your fly. A "low-stretch" fly line is also HUGE help, and often helps to set the hook with less effort on the angler's part.
As for the river fishing... use a shorter amount of fly line as well, and stay in direct contact with your fly by using a shorter leader (nine feet is usually sufficient). By using a longer amount of fly line, you will have more difficulty managing your fly line and fly, as the varying surface currents will cause drag and reduce your chances of a quick hook-set and/or contact with your fly. Striking too slowly will cause many misses as some the best dry fly fishermen will strike quickly when river fishing and simply drop the fly back at the same location if they fail to connect (lakes are a different scenerio). My theory is that if the fish fails to feel the sting of the fly, you have a chance that it will return a second time, but if it has a chance to taste the fly and refuses it, rarely - if ever - will it rise a second time...
Good-luck and good angling...
Todd