The most obvious reason is that we are expecting different kind of outcome or effect to take place but Reiki as pure in nature brought something else. So the expectations were not met, but at this very moment Reiki has stirred or something not so apparent has taken place inside of us which we are not aware. Even if we are, we will not talk about it as most of Us are not comfortable talking about our inner processes.
The other reason could be persons willingness to heal. For e.g. there was a friend of mine who only got the support and affection of her family when she was ill. So it is very difficult for Reiki to heal as this kind of experience is deeply rooted and will present inner resistance to any kind of treatment or therapy that can help.
Then there are those who come for Reiki treatment to prove to us and themselves that Reiki cannot do anything, its a waste of time. They lie on the treatment bed full of tension, ego and skepticism resisting every flow of energy and possibility of inner change and peace setting in.