Light Polution Filter / Photography question....

Had a really clear night here in western Washington two night ago and
had great (though cold at 24F) seeing for this area.

I tried using the Celestron LPR filter for the first time with film
photography. Just tried taking pictures of fairly dense areas of
stars in the Milky Way. Didn't see really see anything through the
filter, so with the idea that there would be some star light there I
took a couple 30 minute exposures on 200 speed color film (all I had
handy).

Frames werealmost totally blank, just a star or two discernable under
the loupe. Is the problem the filter, or do I need to do longer
exposures? Is Celestron's filter not as good as others? When I've
tried pictures without the filter I get the stars but the background
come out greyish.

Any suggestions would be helpful, thanks in advance.

(Oh, using a Celestron C8 NGT - f5, fl=1000mm, if that makes a
difference)