When did you download the patch your using?
Linux IP NAT Forum
NAT 2.2.4 - Not working? (Jason Sustarich)
Keywords: NAT 2.2.4
Date: Tue, 20 Apr 1999 12:15:32 GMT
From: Michael Hasenstein <email@example.com>
See top of forum (download section), there's a later version available with a small patch. Looks like the original version missed forwarded packets, which matches your description of the problem.
Recently Downloaded Newest Version by Jason Sustarich, 4/21/99