How do I add new files to a signed jar without getting the error: JAR resources in JNLP file are not signed by same certificate

I have a web start application which consists of several jar files (packaged into a war file).
I have the certificate that was used to originally sign the jars.

I want to add a new file to one of the jars and repackage. After adding the file to the jar and then signing the jar and repackaging, I get the following error upon launching java web start:
JAR resources in JNLP file are not signed by same certificate

I am sure the certificate is the same. Do I need to go through each and every jar and sign them all again, or is there something else wrong here?

If the only solution is to re-sign all the jars, then does anyone have an ant script already that they can share?

Some notes:
When I add the new file and sign the jar, I am not modifying the contents of the META-INF directory at all. I read somewhere else that you could try removing part of the MANIFEST and/or the .SF and .RSA files - I cant find official documentation from sun that says doing this is valid, so did not try this.

Because of the pre-packaged nature of the application the jnlp file must not be changed if it can be avoided.

Thanks.