If the flies with CDC wings are correctly tied they will catch one fish after another. If the CDC is not properly fixed the flies will catch fish only at the beginnig.

For example, the fly below has the CDC fixed in the classical way like at the F Fly. In time the CDC barbs start to lean back. Instead of imitating a dun the fly ends up imitating a sedge. This is the reason why the fish will not touch it. 

After a few casts and fish the fly will look like in the pic below. 

So no matter how beautiful the fly is in the picture:

Iit is better to tie it as following;

The CDC fixed like this will stay up, correctly  and after casts and fish caught it will keep its original position without leanig backwards.

Another method is to make a few turns with dubbing behind the wing 

I tried several kinds of dubbing untill I managed to form this torax with TL Mad Rabbit Dubbing which I consider the best in this case.

Here are 3 different ways to tie the same fly.The second and the third work better