This is a great topic for me, because I have a hard time letting go of characters. Sometimes it's just that too much time has past and I forget everything that came before. I'd have to re-read the whole series and that's a serious time commitment. Or sometiems, it's because I have too many books out at the moment it comes in and I want to let library patrons have a chance to read it first, but then I forget to go back and grab it and so much time passes. I really do WANT to read these books!
Secret Life of the Immortal Nicolas Flamel - loved this series, but couldn't bring myself to read the last two. I think I'll have to re-read them all and soon!
Percy Jackson - read the first 3 books and LOVED them. Took too long to read the 4th (Still haven't) and forgot everything.
Maximum Ride - I got to the fourth book and really didn't like it - too much environmental preaching. Then I stopped reading it, but I hear it gets better
Song of Ice and Fire - I'm about halfway through Dance with Dragons. This one isn't entirely my fault. I'm reading it with my husband and it's hard to find time to fit it in.
Georgia Nicholson series - Again, stopped at the last two books
Pendragon series - same, last two books
Vampire Kisses: The first vampire books that I read and loved.
Golden Lily: I still need to read this book.
I'm sure there's more, but I just can't think of any at the moment.