The manual just says to bump the head but this does nothing.
John, I also own a 15" trimmer which I bought from early 2016. To date, I am still using my trimmer with great success, and have not needed to replace the "original" string from the bump head. That said; try all the steps below from the manual and see if you can get it to work. If not, then contact Customer Service so they can diagnose the issue, and help you get this to work right away.