Usually the best thing that helps me with writer's block is to take a break from it, watch TV, go for a walk, play piano, go for a drive, etc and then when I get back to my laptop, the story just flows easily.
I'm confident that tomorrow will be an easier writing day!
Thanks for all the storyline suggestions. I hadn't really considered bringing some old characters back until a faithful reader suggested it (thank you :) and I realized that it is a good idea to bring Katie back, so she'll be back in book 6!
Also, the series will be never ending (until there are no book sales, at least).
Thank you for all your comments!