Writing a series is a long and difficult process. There are so many things we must keep straight between each book it can be easy to get lost or frustrated. But tackling the three major problems you will run into will make the process easier.
Character Arcs
How to Establish a Character Arc for Your Story
Creating a character arc can seem like more work than is needed. But if your readers can't relate to your protagonist's struggles, they won't care about the story, and it will seem boring to them.