Rogers' Core conditions necessary and sufficient?

Recently I have struggled once again to see for myself if Roger's six core conditions are enough. Through finding this gem I have come to understand more of Rogers' thoughts on it. 

 I believe many clients drop out of therapy before it has helped or just as it is beginning to help. I believe that this quote from Rogers stating that the core conditions continue over  period of time captures part f what is happening. As therapist we see our clients 'process' if we are experienced and the client is committed to therapy we can sense when a client is going end their journey with us.

 I now see that sometimes clients don't stay long enough for the core conditions to have enough of an effect for them. This is why I believe that the core conditions are always necessary but not always sufficient, especially near the beginning stages of therapy.

'No other conditions are necessary. If these six conditions exist, and continue over a period of time, this is sufficient.The process of constructive personality change will follow'
 Rogers C, (1957) The necessary and sufficient conditions of psycho therapeutic personality change. Journal of Consulting Psychology 21 p96