not all layers appear in "Manufacture -->Artwork"

Hi,
I have a 6 layer board, 3 planes, 3 routing layers, and when I try to
produce the artwork files, only the Bottom and Top layer are there for
checking off.

When I hit the Create Artwork button, I get the following errors:
DEVICE-TYPE GERBER6X00
OUTPUT-UNITS INCHES
FILM-SIZE 24000 16000
FORMAT 5.3
ABORT-ON-ERROR NO
SCALE 1
G-CODES YES
OPTIMIZE YES
STATIONS-PER-WHEEL 999
COORDINATES ABSOLUTE
SUPPRESS-LEAD-ZEROES YES
SUPPRESS-TRAIL-ZEROES NO
SUPPRESS-EQUAL YES

WARNING: database resolution exceeds the resolution of the
output coordinates (see FORMAT in parameter file).
Some roundoff of coordinates may occur unless the number
of decimal places in the output file is increased.


ARTWORK Error reading apertures.



Anybody knows what is causing the problem? Thanks