There's only 2 series so far that I have dropped and not got back in to.
First is Little Apocalypse. I have read 10 volumes of that and haven't read any more for about 6 months. I took a break on it as well after 4 or 5 volumes as well. I find that the way that the series is written with introducing 3-4 new girls every volume to become overwhelming. Way too many characters to remember and I find that the structure of the stories are too similar so that all the volumes seem like the same story with a different setting. I'll probably get around to finishing it at some point, but it's very low on the list of things to read.
The other is Ascendance of a Bookworm. I've read the first 2 volumes, and I have the 3rd as well, but I found it hard to enjoy the series. I don't particularly like Myne as a character and the story it self hasn't been very interesting. I don't usually mind a slow story, but it needs to have something else to go with it like humour to keep me interested in progressing the story, but this one hasn't really done it for me. I might get around to more of it at some point, but it's even lower on the list than Little Apocalypse.