Instrument skills are all 'C' ranked skills for Bards, which means they have a certain equation associated with them which determines their max skill at each level. Here's a link to where you can find all kinds of info about different skill rankings and equations: http://www.ausystem.org/~aushacho/gbox/ff/skill-e.html
FYI (if you don't want to deal with skill charts and whatnot), at level 6 all three skills should cap at 19, then 21 24 and 27 for levels 7 8 and 9 respectively.
Good luck =)
Sylph server Edited, Tue Dec 30 18:56:28 2003 by Darakar