This is a question that I feel is out there, but most Reiki practitioners don't have a solid answer. It's also something that can be a hard sell to new clients who have never experienced Reiki healing, or any energy healing modality.
So why do some cases take more than one treatment? The simple answer...entrainment.
Entrainment, in physics, is the process whereby two oscillating systems in close enough proximity will, over time, assume the same frequency. In other words, if two pendulum clocks were hung on the same wall, they would eventually swing at the same rhythm. If we think of the clocks as each having a frequency, the higher frequency will come down, and the lower frequency will rise, so that both frequencies are the same. So what does that have to do with healing?
Well, everything in the universe has a frequency. This is the Law of Vibration. The governing law for the Law of Attraction. "Everything moves, nothing rests." Everything that exists, exists in a different rate of vibration and harmony. When we are healthy, our vibration is higher and in harmony. When we are in a state of dis-ease, we are vibrating at a lower frequency and disharmony.
Reiki is pure universal energy. Unconditional love. And Love is the highest vibration in all the universe. When one receives a Reiki treatment, the goal is to increase your vibration so that your body can heal itself. By cleansing the aura and balancing the chakras, we are able to bring the body into a more harmonious state and higher frequency that will lead to healing of the physical body. But because of entrainment, this harmony doesn't last forever. Because inevitably after a session we then return back to a world that is at a lower vibration. An individual reality that has been created through fear, anger, guilt, jealousy, depression, or anxiety. Emotions that vibrate in the lowest frequencies. These are the emotions that have created our dis-ease. So for complex and chronic cases, some healing will occur after one session, but then you will come back down to balance the lower vibratory rate of the reality that has existed. Gradually, with multiple healing sessions, not only does the body begin to heal fully and completely, but our reality begins to change and vibrate in harmony with health.
Dis-ease is cumulative state of being that isn't created in a day, and the same goes for healing and health. While each individual has different healing needs, it's important to remember that healing takes time, and this is the reason I always recommend at least 3 Reiki sessions for any client.
So do yourself a favor and give yourself the time and love to heal.