Unfortunately, the 742's were called Jammomatics because of this exact problem. They have weak extractors. If your extractor isn't actually broken, you may be able to resurrect it by making a chamber brush out of a shotgun-sized brass brush. You'll have to do a little trimming to make the shotgun brush match the size of your bullet casing. Screw it onto a short piece of cleaning rod and bend the rod at an angle so that you can insert the brush into the chamber and work it in and out and twist it too.
Clean the chamber till you think it is clean, then clean it again. Use a VERY light coat of good gun oil and then wipe it off with a clean cloth. Now try to extract a shell. If that doesn't work, you'll need to find a gunsmith that is willing to work on 742's (that in itself is very hard as most don't want to touch them). Replacement extractors are hard to come buy too.