Re: Problem on the G and the B string
Get the fret level checked. You may have uneven frets at the top of the neck. A lot of guitars will ship from the factory with very little setup work done on the frets, and you can find that a new guitar may need the frets filed. If there any high spots, the strings might be touching them when they vibrate, which would certainly cause them to die out quickly. And the further up the neck you go, the steeper the string angle is, so it could explain why it only happens after the 15th fret.