A couple of suggestions:
First, as several people have already mentioned, have the plates
facing downward, so the etching crud falls out of the pattern. I've
found the easiest way to do this is to use double-sided tape to
stick them onto those foam trays that the grocers use for meat & etc.
The tray floats around on the top of the bath, and your piece stays
on top of the bath, facing down.
I recently taught a couple of classes in ferric chloride etching,
and the thing we found most effective for boosting our etch speed was
to have the bath seriously agitated. We used a magnetic stirring
unit, with the plates floating around on top of the bath, and could
chew most of the way through 18ga brass in about 3 hours with a
fresh batch of mordent.
PS--> always mix chemicals with distilled water. Your tap water might
be great this week...then again, it might not. Distilled water is
always the same.