Bob
Frets being too low is a very uncommon problem.  The only way they could get
that way is if the board warped and someone flattened the tops of the frets to
make them level, and that would result in some being tall and some not-so-tall.
I guess you can never discount ignorance as an explanation for things like this,
too.

First thing to do is to set a 12” metal straight edge from the top of the first
fret down the board and the other end will be on about fret 13 or so.  With a
feeler gauge, you should get about .005 -,010 between fret 6 and the straight
edge.  This is called relief and is essential to getting the notes right.

If this is good, then measure between the top of the 12th fret and the bottom of
the strings at either side - the 2 E strings.  You would like to get about
1/16”+ to 18” or so.  Too low and the strings are too close to the board.

You can also use your feeler gauges to see how tall the frets are by laying then
between frets till you find a combination that equals the height of the fret.-
you would like to have a minimum of about .025 and could be as high as .045 -
measure a couple

Take these 3 measurements and get back to me.

Ed Minch