First of all, tell me a little about your series and introduce us to the sequel(s).
My Guardians of the Tide series follows the life of an orphan named Aust on the ocean world of Nix. 
The world of Nix is broken and so are its people. Three roving moons devastate the planet with massive Moontides that follow in their wake. What remains of civilization, clings to life on migratory floating villages. But as a world dominated by the element of water, change is inevitable, and that change is spiraling toward destruction. 
Amidst this chaos, Aust strives to protect a band of young orphans from an endless array of dangers. The events of Moonrise’s Call (Guardians #1) were just the beginning. A new threat approaches, its song emanating from deep within the Nether Sea. 
To survive, Aust will be forced to cast aside old enmities and forge new alliances. But learning to trust is not so easy for the Orphan King, and for good reason.  
Aust’s journey continues in Song of the Rising Deep (Guardians of the Tide #2).

Do you find that most of your readers continue to read the whole series? Why do you think that is?  
As long as I can get someone to read the first in the series, they usually continue through. 
I would like to think it is because of the way I create the opening for the next leg of the adventure as I close out the previous journey. I like to give my readers a taste of what awaits them in the next book. 
Plus, I try to write likable characters that connect with a reader. Readers care about what happens to them and are eager to find out more.

How difficult is it to add new characters in a sequel into already established relationships? 
Every character interaction is different. I think it is more the personality of the new addition combined with how their story interacts with others that might make it difficult to weave into a group. It’s more than simply adding new characters. 
Thankfully, I plot out my new characters and their connections/future connections with the main characters ahead of time. So I already have an idea how they will all fit together. 
However, most authors will tell you, just because you tell a character to do something doesn’t always mean they will. Sometimes a personality develops throughout the writing process. until the actions you planned for them no longer fit the person they’ve become.

Is it difficult to continue with worldbuilding for a world you have already built in book 1?  Do you find it easier to switch locations for the sequel and start again with worldbuilding? 
Not really. 
When I design my worlds, I design for the series. I may have to flesh out a few details along the way, but before I set out writing, I often know all of the main stopping points and locations of major plot points throughout the series. 
The worlds in my books are more than just the land that an adventurer travels through. They are part of the story, influencing the characters and directing the plot. The worlds are intrinsically linked to the plot, so as I develop the series plotline, the world develops as well.

Have you ever been stymied by a worldbuilding or plot detail from book 1 that is very inconvenient to deal with or write your way around in subsequent books? 
No. Once again, this comes down to my world building and plotting ahead of time. I am not saying it won’t happen in the future. But for the most part, the foundations I build before writing these adventures are usually strong enough to accommodate any questions or details that develop later on.  

Would you say your craft has improved with the subsequent books?
Definitely. The more I write the better I get. I develop more as an author with every sentence I craft and every story I complete.
The deeper I get into a series, the more I connect with my characters and learn who they are. I gain a better feel for their voices and convictions. I fall in love with more fascinating imaginary people and am drawn deeper into the peril of their story. 
I feel the same pattern follows with my Guardians of the Tide series. It begins as brief glimpses of Aust’s life and the struggles he faces. He isn’t on a grand epic journey. Yet! He is just trying to survive and protect those he cares about. But those struggles are preparing him for something greater, something epic.   
Hopefully, I can follow the same character arc with my writing. 

Do you have all the timeline planned for the full series?
Yes. I have the last 3 books in the series plotted out. I am primarily writing book 3, Whispers of the Shadow Sea, right now. However, the last 3 books are so intertwined that I am developing Arc of Harmonies (#4) and Cadence of Oblivion (#5) along the way.

Do you have any marketing tips for sequels? 
I wish that I had some secret to share. I am still trying to master selling the first in a series. I know the first book needs to lead toward the sequel. Give the reader a desire to know more of the story. And make sure there is a link for the next book available at the end of the first. 
I plan to try the rapid release plan with the last three books of my Guardians series. I have seen a few authors getting good responses by releasing their books over short intervals. So I am devoting this next year to finishing the series and doing continual promotion for the series with my fanbase along the way.