In the Bhagavad-gita 4.42 purport Srila Prabhupada states that the genuine spiritual master is in the disciplic succession from time immemorial. What exactly does this mean?
In the same text Srila Prabhupada says that a bona fide spiritual master never deviates from the instructions given by the Supreme personality of Godhead. The point is that in ISKCON we have seen many gurus falling away from the path, does this mean that they were not genuine?
At the end of the Srila Prabhupada Lilamrita the author says that Srila Prabhupada is still in charge of everything. What does this mean?
A genuine guru is a disciple of a genuine guru. The original guru is Krishna who begins the chain of teaching. This process has been going on since time immemorial.
In any field of activity, a person may perform their duty properly, but then deviate or neglect that duty. Some devotees did good service as gurus, but then experienced some difficulty. This difficulty does not invalidate the good service they did.
Srila Prabhupada still leads us because ISKCON leaders accept his Krishna conscious teachings and example as their guide and authority.