Here's how to use a Doyle Zenith main
Phil, I imagine you have a Doyle Zenith vertical batten furling main as provided by Hunter - at least we did on our 2005 H41 and when we first got the boat (new), we had jams too. Turns out we weren't unfurling and furling it correctly, as we discovered when the head of the local Doyle loft took us out sailing to show us how. After he showed us proper procedure, never a jam again in the 7 years we had the boat.
Here's the link to Doyle's instructions for their furling main with vertical battens. Be sure to follow them explicitly
especially steps 2 and 4 on this page:
http://www.doylesails.com/sails-cruising-coastal-mains-zenith.htm
Don't let the furling winch at the mast freewheel when unfurling, and furl in with a tight furl.
Be sure to click on the links at the bottom of that page to obtain even more detailed instructions.
To fix your current jam, just try working it out inch by inch by pulling out and down on the leach, and yes you may have to go up the mast to exactly where the jam is to work it out of the mast (we never had to). After that, do a very tight furl-in and you should be set to go.
Brian