I don't know what you can do to fix your sweater if it is already completed, but here is an idea for another time.
Check out short rows for adding length to the back. I have never done it for that reason, I have added a few short rows on the back of a sweater, but I thought it was more to help the shoulders fit but maybe it is done the same way to keep a sweater from riding up.
Here is a site that speaks of using short rows to keep a sweater from riding up in back. LINK
Scroll down to where it has the heading, "Torso Darts".
You will need to do some more research on the subject but I think this may be the answer to the problem. Good luck.