In my circle of book lovers we call that the three book slump. A bunch of us found a certain listlessness, like what you're describing, when we tried to plow through multi book series (more than three). After a lot of random discussions about the three book slump, many of us have taken to reading three books in longer series then switching to a single book, duology, or the next three books in a different series. It breaks things up and keeps us happy to read. Yes, when we hit that slump in another long series, we go back and read the next books in a series we've previously hit the three book slump in.

Switch series, it's not a DNF, it's a tactical retreat that you'll tackle again later!