I tried out an Ibanez Artcore today. After the salesman plugged it in and "impressed" me with his slap riffs , I sat down and proceeded to tune the instrument using harmonics. They resonated as one would expect until I got to the 7th fret of the A string, and the 5th fret of the E string, from which I got an unsatisfying thud. What seems especially odd is that the 5th fret of the A string rang out, while the 7th fret did not. What could cause this?