We have been trying the "sin mosca"  liquid out now for a month with no visible effects.  We are not inundated with flies, but they are no different in the sprayed area than out of it. We did two seperate areas, one in the pergola next to the house where we eat, and one in the summer house further up the garden. Neither is any different. We sprayed very thoroughly in all areas.
We live in Tivenys, there are trees about, but we are not up any mountain, maybe the answer is to cut all trees down, then spray..!!
We had the liquid bought for us as a present, and the lady who makes and sells the stuff knows where we live....there was no mention when she sold it to our friends that "it doesn't work in Tivenys etc..." maybe the label needs to have some further instruction...??