When fly fishing for pike, I've noticed that the way the fly is retrieved can make a big difference to what you catch - or don't catch! I've seen this in practice many times when fishing with other anglers. One often catches more than the other and, although there are other factors, retrieve style is often the only real difference in the approach. For this reason, I will vary my retrieve style until I find something that triggers them. As with fly fishing for any species, there are a few standard retrieve styles. Most pike fly anglers will 'strip' the fly by pulling back a certain length of line in a regular pattern, This can be varied by the length of line retrieved in each pull, the speed at which it is pulled back, and pauses of various times between each strip. Then you can use a 'figure of eight' style which is a gathering of the line between the thumb and fingers so as to give a constant line speed. Once again it pays to vary the line retrieve speed and make use of pauses during the retrieve. Then there is the 'roly poly' retrieve which is performed by holding the rod between ones legs while stripping at high speed with both hands.
There is a lot of variation in these retrieves, and I often mix them up to give a lot of variety to any fish that might be interested. One of my favourite retrieves which I never see others use, although I'm sure there are some that do so, is to make use of my rod tip to accelerate the fly so it really darts at high speed. It simply involves getting into a rhythm of an accelerating strip, followed by a line gathering strip. When making the accelerating strip, I pull line back with my retrieving hand whilst moving the rod tip vigorously to one side. This is quickly followed by a shorter strip that takes up the inevitable slack line. It's a bit like using the hauling technique when casting to increase line speed except that you are doing so with the fly underwater. Again, you can vary this speed and the distance that the fly darts, and don't forget those pauses.
Catching pike on the fly is usually not that difficult when they are active. When they are not, the challenge is to find out what is required to make them react, and this, to me, is the most rewarding fly fishing of all.