Why do some who start to practice the Krsna Consciousness process become truly happy and some become or stay misserable?

The answer may be more simple than expected. Those who become not only happy when practicing Krsna Consciousness but truly ecstatic are those who realise that they have found a way to be themselves. After all, cheating oneself automatically means cheating others, and to be a pretender is an extremely mentally and physically demanding way of life. And so, who came to understand that in Krsna Consciousness there is no need to be a pretender rejoices and embraces the procession of purification Srila Prabhupada offered to us.

Contrary to popular belief, those who are and remain misserable are those who understand the Krsna Consciousness process as another way to be what one is not. The game of cheating is basically the same; just this time one can pretend without presenting any tangible result, as is mostly the case in karmi society, where performance of any kind is judged in an uncompromising manner. One just has to chant superficially, assume the role of a pseudo-rennounciate, and accept a costume similar to that of those actors playing in a theatre. Then one has to find enough fools and innocent ones who may believe what one tries to pretend.

One can cheat many for a short time and a few for a long time, but one cannot finally cheat everybody all the time. And so, guessing that, a pretender is in constant fear of being exposed by those who came to join Krsna Consciousness for the reasons mentioned in the first portion of the answer.

Looking into the history of the ISKCON movement, we find the struggle going on between these two opposing parties, the ecstatic lovers of Truth and the misserable pretenders. Srila Prabhupada expected us to impress the public with our purity, sincerity, and bliss. When pretenders assume leading positions, society, repulsed by their appearance, is not attracted, and those who are serious seekers of the Truth retreat in shame. Finally, preaching stops.

This is the future we should try to avoid. It is easy when constantly mediating about the ways Srila Prabhupada presented Krsna Consciousness, making even the most wretched and depressed men to ecstaticly dancing preachers.
By following him and meditating on his example day and night, we may become freed from our inherent hypocrisy and misunderstandings about ourselves.
We should stop being pretenders. Not being forced to be a pretender is a luxury that not many have.