If you do not apply several coats of glaze the pieces will often come out
looking dull. Some underglazes seem to require more glaze on the top of
them than others.
Was the Laguna clear glaze on all the pieces? Or only on some of the
pieces? Laguna makes a medium fire cone 6 clear glaze which is different
than cone 06/05. Could you have purchased the wrong clear glaze?