Writer's block is a mental, non-medical condition where the particular author is faced with a loss for words or ideas in not being able to continue or expand upon his or her tale. It is unique to writers and that is why it is termed 'writer's block'.
When I was writing the first part of "The Mythical Emblems of Gragodon", I used to literally get inspirational flashes at different times of the day and night. Writer's block did not affect me much then because the introduction of fresh and newer scenery and the varied characters making up the story afforded me ample food for thought. When I came to the later volumes of this epic tale, because the story was a continuous flow from one book to another, I found that the occasional bout of writer's block came at more frequent intervals.
Occasionally, my family members came to my help and offered helpful hints which, more often than not, helped spur my imaginative inclinations and ended those blocks for the moment. Many a time, I was at a loss for words, wondering what the ensuing sequence of events would turn out to be. I had read from published authors that a good remedy for writer's block was to just take a few days off from the writing process. Perhaps the brain needed to rest from the continual need for fresh ideas. I was also feeling tired because I used to burn the midnight oil more often than occasionally. Coming back to continuing my writing after some days, I realized that I had not been able to proceed with the story.
At times, finding your own unique path is the best approach to solving your particular problem. Feeling that I had had enough 'free' time, I started to ponder deeply over my storyline. The more I continued with this approach towards finding an outlet for more inspirational ideas, the faster I was able to get fresh leads to continue my writing. It was therefore not the days of rest which I had taken from my writing, but looking at my particular story from a different perspective or angle, which cleared my block. I was thus able to complete the six volumes of "The Mythical Emblems of Gragodon" with my spirits still in fine form. I would therefore definitely suggest this approach towards overcoming 'writer's block', because it had worked well for me.
I once read Isaac Asimov's, Opus 100, his 100th book describing the other 99 and how he wrote them. Anyway, he said that he had several stacks of manuscript that he was writing all the time. He would write on one book for a while, and then as his muse would change he would either start a new story or or go back to one he had already started. Eventually he would finish one and have a completed book or short story. Since he had several things going at once, if he encountered a block with one, he could always pick up on another that he had been thinking about. This approach made him very productive and able to work on all the ideas he had.
I've adopted Asimov's approach and it works quite well for me. I don't set deadlines for the projects I have underway. Every once in a while, I get very busy and finish a project that I've left aside. Occasionally, especially with a short story, I write the whole thing in one sitting.
To write a long series of books on one topic, like you have, requires discipline I don't think that many possess. I know I don't.
Ron