This is how I write series books, because I'm a reader first and I know what I like to keep me interested.
So firstly, be sure to have a bad guy or conundrum that runs through 3 books, solved at the end of the 3rd book. Make your series themed with underlying themes every 3 books. The entire series can be 9 books long, or even more! The hero solves a case in every book, but for 3 books there's also another plot-line haunting the MC.
The reason shows like Orphan Black and Extant don't work for me is because the only theme is discovery and anything can change. The story goes no where and we don't care about the bad guys because they get killed off all the time.
For example, OB is about discovering more about why they were cloned, but that's boring because they were cloned and they could do stuff with that. They could spy for people., impersonate, they could be a group of investigators!
Extant is ridiculous. The storyline changes too often because each discovery is dead boring. They could have the robots do things! Like solve crimes or grow with artificial intelligence aspects!
The reason season 1 of Miss Fisher's Murder Mysteries was so good was because of the continuous murderer from her past. Also the building romance between her and the detective. Seasons 2 and 3 were okay, but they would have been better with new underlying themes throughout. They gave the detective a divorce so that should have freed Phryne and him to hook up, but they never did because the writers wanted the romance to go on, because everyone knows viewers (and novel readers) get bored with a relationship once the characters find 'true love' and their 'happy ever after'. They could have driven them apart and brought them back together in the end.
My problem is sticking to rules. When I write I don't outline, I just write and the characters take me where they want to in a story. Well, that has to change. I need to start with the above aspects and insert them throughout, whatever it takes. Like Sherlock Holmes and Moriarty; the bad guy who doesn't just get caught in the first story. Moriarty plagues Holmes throughout many stories.
(By the way, Moriarty is a real name. When I started working with a woman with the last name Moriarty I asked her if that was her real surname. She said, "yes," and nothing else. I figured out real quick that she gets asked about her last name a lot, so I never talked about it to her again. I'm nice like that. Haha. It's a wicked cool surname to have though, wish I'd at least told her that.)
Another show I started watching is called Contiuum. When I read up about it, before watching it, I thought it was going to be a show that would involve a gripping plotline. I'm not going to say what I thought it was going to be like, because I'm keeping it as my own idea for an SF series! Whatever the case, it is not like I thought it would be and I won't even be continuing with watching season 1.
A final TV show example of a series with a good theme is House. The old medical show. Viewers were kept interested in the problems of the characters throughout every season, that was the addictive hook. Without the character's problems it would have just been solving medical problems and wouldn't have anything to keep me as a viewer coming back to find out what happened to the main characters.
It's tricky with book series that aren't yet complete. I'll only give examples of complete series that have worked, because I never ever demean books I don't like. I'm a writer, I know what it means to be an author of books that get both good reviews and bad. There's just no necessity in writing bad reviews. It goes with what we were taught as children, but fail to remember as adults just to be plain nice. "If you don't have anything nice to say, don't say anything at all."
So The Hunger Games is a good series. There's an adventure 'solved' in each book, but the entire story isn't completely told until the end of the 3rd instalment.
I'm currently enjoying The 5th Wave series. Well, the 2nd book was a bit confusing, but I'm hoping desperately that the 3rd and final book will clear everything up excitingly!
I will say there was a series of 9 books where the author ruined the entire series for me with the final book in the series. That is devastating for a fan and reader. Years and years of exciting reading, anticipation of the finalé, only to have all that anticipation crushed. It made me wish I could re-write the final book myself, but I don't have time for fan fiction.