Most mendolins have movable bridges, held in place by the strings. Check that the bridge is in the right place (starting at twice the distance from the nut to the 12th fret, then fudged to account for string stretch as you fret).

If you have an electronic tuner, check to see that the 12th fret note is one octave above the open note for the same string. Another check is to use the harmonic at the 12th fret, checked against the fretted note to see that it's the same note.

If the bridge is in the right place, and the notes are still out, then the frets are upgephrunzeled. If moving the bridge fixes the problem, well, then you've fixed the problem.



Songbob