There is a theory as to why the neutrinos are missing. There is not just one kind of neutrino, there are a few differnt types. The theory states that in extreme conditions, such as those that exist in the sun's core, It is possible for the neutrinos that we can detect to be changed into other types of neurinos which we can't detect yet. The normal neutrino can be chaned into a mu neutrino or a tau neutrino. This research is fairly new and there is no real proof of it but models suggest that it may happen. If it happens it would explain the missing neutrinos.