Fairly early I distinguished between writers who are for one reason or another losing their skill (Terry Pratchett alas) vs series that went to pot. We mostly stuck to that throughout.
One panelist thought it was always a failure in world-building: you hadn't built up your world with enough complexity to undertake the tales. I didn't think so, because my commonest examples have been books where the author has to undermine the last book's happy ending. After a world-imperiling cataclysm, you need another one, or the story will seem anti-climatic, but the world needs saving again? Or the hero's personal issue, resolved at the end of the book, returns full strength at the beginning of the next.
Dune's sequels were cited as a failure where the world wasn't big enough, and Herbert didn't expand it for a sequel.
Still, it can also be dangerous to make the world too full. There's the danger of clutter.
Comic books face a particular peril, as they have unlimited series. Part of the problem is that a longer story is not just a short story drawn out. It needs a larger structure. An infinitely long series would need an infinite architecture. Much agreement that you have to be able to give up a series. One panelist thought that going back was a peril, but in my experience, going back to a series after writing other works helps refresh the writer.
I talked about changing characters. Let your character resolve his personal issues and then shift to another, who hasn't. Edgar Rice Burroughs made a mistake, usually, when he stuck to a hero who had met, clashed with, wooed, and won, the maiden; he needed the boy-meets-girl, boy-loses-girl, boy-gets-girl to give structure to his story.
The author who drudges on without inspiration. L. Frank Baum in Oz. Then Doyle drudged on with Sherlock, and he always thought it hackwork. Some writers do their best with work that they think beneath them, or just light and frivolous. (Hmmm. Didn't come at the panel, but sometimes the series jumps the dragon when the author decides he can, after all, put Deep and Serious Thought into a formerly light series.
I'm afraid that some panelists sometimes used the panel as a chance to dump on series they didn't like. Especially since there was some talk of having jumped the dragon from the start.
Jumping the dragon vs. a bad book. It's one reason why we keep on reading a series after the jump, because we hope the book's a fluke. We may also care about the characters or want to see something. Plus habit. But sometimes you have to tear yourself away.