Even though the process of Krishna consciousness is so blissful then why do senior devotees sometimes fall down or leave the movement?

Yes, it is true that the process of Krishna consciousness is blissful. However one should practice in right attitude.
The falldown of senior devotees should not be considered ordinary.
There are two reasons for this :
1. If one commits offense toward Krishna’s devotees and
2. It is the plan of the Supreme Lord.

Although sometimes they may go away from the movement for some time, Krishna never forsakes them.
The progress or advancement they have done on this path is never lost.
Sooner or later they shall come more closer to Krishna.

According to Lord Krishna, even if someone commits most abominable activity, however he continues to take shelter of Krishna through His devotional service, he should be considered saintly.

Sometimes it is better to be a fallen devotee and remember Lord Krishna with great intensity than to be a so called devotee and to become proud.
As Srila Prabhupada rightly gives one important example: A sincere sweeper in the street is far better than the charlatan meditator who meditates only for the sake of making a living. (BG 3.7, Purport)