The WTS doctrine of the "faithful and discreet slave" (FDS) is in direct contradiction with its doctrine of "the light getting brighter" and the "wheat and the weeds." Like many JW doctrines, these look fine from a distance. But if you take the time to examine them in more detail, they fall apart. Here's something I put together on this matter:
Background
1. The light gets brighter
As you know, Witnesses explain away their silly old teachings and false prophecies by means of the "light getting brighter" statement from Proverbs 4. Thus, according to the Witnesses, we have a much greater understanding of the Bible now than did the friends in the days of Russell, Rutherford, and Knorr.
2. The 1,900 year-old Faithful & Discreet Slave
The Witnesses teach that Christ appointed the FDS in 33 C.E. There has been a FDS on the earth at all times from 33 C.E. until today. On several occasions, the literature has stated that one generation of the "slave" fed the next generation—including during the post-Apostolic period up to Russell's time.
While the Watchtower has occasionally included articles about various dissident groups during the "dark ages" in Europe (e.g., the Lollards, Waldenses, Levellers), there has been no statement affirming any of these groups as comprising the FDS. Also, there is no evidence that these groups fit into any sort of unbroken chain of faithful slaves providing food to the next generation at the proper time.
Further, the Society teaches that Russell's group was not an offshoot of any group or organization. Rather, one publication characterizes Russell as reviving the great truths of the Bible. Statements such as this insinuate that Russell simply prayed for guidance and was led by Holy Spirit to reject the popular teachings of Christendom (Trinity, hell, etc.).
The Contradiction
So, the Society appears to have adopted a contradictory position. First, there has been a FDS on the earth since 33 C.E. Second, C.T. Russell was part of the FDS. Ok, so far so good, right? Here's where it gets dicey for the Watchtower. C.T. Russell's group was not an offshoot of any group or organization—rather, they studied the Bible free of any preconceptions from other religious groups. So, how does this square with the FDS dating back to 33 C.E.? Shouldn't there have been a FDS group on the earth in the 1870s to give Russell the food at the proper time? Why did he have to come up with it himself?
Also, going back to Proverbs 4, who had a clearer understanding of Bible truths—Russell, or the 1st century Christians? I think it would only require a quick glance through the Studies in the Scriptures and some copies of Zion's Watch Tower to answer that question. So, that begs the question: why didn't the light get brighter between 33 C.E. and 1879? It looks like it got darker, doesn't it?
A Flawed Counter-Argument
An astute Witness would likely counter this argument by raising the parable of the wheat and the weeds. The Watchtower teaches that during the post-Apostolic period until the "time of the harvest" (which JWs believe parallels the "last days"), it would be difficult to distinguish true from false Christians. But, according to Witness theology, once the harvest period began, the wheat and weeds would be readily distinguishable.
This counter-argument is like a Monet painting—it looks good from a distance, but when you examine it up close, it's a mess. In particular, this counter-argument is at odds with the concept of a FDS class existing from 33 C.E. down to modern times. In order for the two doctrines to coexist, then at some point members of the FDS had to stop believing certain "true" doctrines and start believing "false" doctrines. Picture this: it's the 2nd century and the members of the FDS meet and decide to start believing in the Trinity. Would Jesus consider these people to be "faithful" and "discreet"? Do JWs believe that something like this happened? Probably not; but I would love to hear an alternative explanation.