New wheelset, spokes losing tension
If you are looking to learn about wheel building, google search Sheldon Brown. He has a website that is phenomenal. I highly recommend it. I learned everything I know about wheel building from a couple hours of youtube videos, the park tool bike mechanics book, and his website.

As for your specific question, I think overall the spoke tension throughout the wheel sounds like it is not enough and after tightening and truing whichever side it is that you are having trouble with I would do a quarter turn on each spoke in the wheel and ride. If it loosens up do another quarter turn on each spoke. With a properly laced and trued wheel with spokes at the proper tension you should never have issues with the spokes loosening.