If you are struggling the problem is probably not fly choice but either location, presentation or (especially ) confidence- or all three. On most stillwaters ,if you fish a floating line with a couple of buzzers , a GRHE nymph or damselfly nymph you should get some takes. Fish as long a leader as you feel comfy with - at least 12 ft and more if you can, especially if you are using a dropper. With buzzers , look for a nice L- R wind (if you are right-handed) , cast across the ripple and let the flies fish round in an arc , with either no retrieve at all, or a very slow and erratic figure of eight. Fish what you think is slowly and then slow it down by half again! Some like to use FAB and blobs on floating line fishing but I'm happy with a well greased leader - which you need to watch like a hawk as takes can be very subtle draws which you don't feel.
For a damsel you can use a faster retrieve - a slow-medium figure of eight is ideal.
It's often worth trying a Ginked up Hopper too - chuck it out and leave it static , and don't strike too fast. Unlike when buzzer fishing, when your strike needs to be instant .
Look for activity and fish close to the wind . Rainbows tend to keep on the move so can pass through your spot before disappearing .
And I repeat - it is nearly all about confidence . When you start fly fishing it seems like some magic trick but the more you do it, the more you will realise that it's not only a great way to catch fish , but a very effective one