Sorry for a bit of offtop...
I'm not sure... but I've just thought that all (or, at least, some) of this may be connected with those fact, that you were featured several months ago.
I've noticed that the activity and behaviour of readers depends on the way, in which they have come to read your work. I myself wasn't on the main page, so I can't tell much, how the readers, who have come from it, behave. But I have a suspicion, that they may be less involved in general, that the readers, who have find your work by themselves.
I suppose that the readers, who just came to the main page and click on random featured novel, may be less interested or have wrong expectations, when they start to read it. While those people, who have started to read it consciously, already knowing that it met some of their criteria, may feel more involved from the beginning. For example, if they have read its description or recommendation about it somewhere else, or have found it by some search, which they performed themselves - they already know something about your work and I suppose that they will drop it with lower probability.
Also people, who find novels more actively, probably are more interested in reading in general. So maybe, they stop visiting site and turn into dead subs with lower probability, than people, who just randomly click on things from the main page.
There are just my clues. I can't be sure about them. Maybe more experienced authors can say, if I'm right or wrong, with more confidence.