I don't do much trout fishing, but have also learned that presentation is often more important than the pattern being used. This actually applies to most fishing, not just with flies.
It's also usually important where you cast, especially in moving water. Generally, trout or other species won't want to fight a strong current to get to a small meal. Not enough gained from the energy expended. I've had many instances where I found that I had to make repeated casts to get a strike. My theory is that until the strike happened, I had either not placed the fly in a position to be taken without excess energy being expended, in other words the "strike zone", or the fish was not actively feeding & my repeatedly showing it the fly annoyed it enough to strike. I think that both situations happen often enough, but we don't really know why the fish strike. Of course, there is also the mistakes in presentation, such as drag on the fly that may be alerting the fish to the fake. Line control is critical for a proper & natural appearing presentation, and we don't always do everything correctly.
As far as what flies to use when there's no hatch, that may depend on the water your fishing and time of year. I usually will go to either terrestrials, nymphs or streamers for searching. I like terrestrials in particular because topwater is more fun to use, but also because slight mistakes in presentation are not usually critical. When a beetle, hopper or ant fall to the water, it's not always delicate, and trout & other species are used to that.
I fish for stream bass much more than for trout, so have become accustomed to using large nymphs & streamers. They do however also work on trout.
Hope this helps!