First of all, it often takes 2-3 days of hatching before the fish really turn on to the danica, and even though the insects are large (and thus easy to spot), presentation is still imperative, and it's not uncommon for especially grayling to lock on to a specific stage in a heavy hatch.
I often find that general flies like a Fremch Partridge is effective during the first part of the hatch, but simply doesn't catch fish after a week or a week-and-a-half. Then more precise imitations are necessary and also more precise representations of the different stages of the insect's lifecycle.
And during last part of the hatch, good luck catching those fish that are locked onto spinners during the evenings. The rise form is easy enough to decode - slow, very slow rises where the fish very rarely exposes it's size, unless you're lucky enough to find one that head-and-tails, which is absolutely fascinating to watch. The dead or dying insects are also often present in good numbers, so the prey is also easy to determine.
But it's when they lock onto dying spinners the trouble begins. Danica spinners often vibrate their wings at a very high frequency causing ripples in the surface and sometimes fish will really lock onto this behaviour and it's more or less impossible to imitate. These fish are just really hard to catch and I haven't really cracked the code. Sometimes a French Partridge will work - I assume the large hackles somewhat imitates the behaviour. On other occasions, a very small twitch of the rod at the precise right time can fool the fish, but I often end of spooking them rather than triggering them.
But it's now - they're hatching like mad and the fish are on. I haven't found any proper fish yet, but next week I will!
Have a great weekend!