How do you know if you've had a successful session?

First of all, there is no such thing as a bad session Melomind™. 

As long as you are aware that you are performing a neurofeedback exercise, you are working. You're even learning while you're in an active state.

Be aware that each of your session depends on the state you are in before you start your session. 

Just before you start your session, there is a step called the calibration step. During this step, a measurement of your alpha waves present in your brain is taken to determine the level of exercise. The goal is to produce more alpha waves than you had before the session. 

Your percentage of deep relaxation time will therefore depend on this step. If you were in a relatively calm state before the session and during the calibration step, it will be more difficult for you to reach the state of deep relaxation because you already had a significant amount of alpha waves in your brain.

On the other hand, if you are in an active state (eyes open, talking, moving...) during this calibration step, it will be "easier" to reach the deep relaxation state.

It is therefore not possible to compare two sessions where you were not in the same state during the calibration step.

For more information on the calibration step, you can read the article What is the purpose of calibration?

You should not only focus on the results at the end of the session but also take into account your feelings.