I think that the Jitter is acting as a wrapper in this case, preventing muji from packing the file. Thats why I suggested cexe. I have used it in the past to compress vb.net exes. Cexe decompresses the exe onto the hard drive and then runs it, making no changes to the intial exe, while other packers change the PE header, making the Jitter not want to run due to an invalid checksum, crc, or whatever on the PE header.
BTW muji, try dling cexe from here:
http://protools.cjb.net/
I dled from there and packed a custom c# prog I made. It worked fine, so it should also work for you.