This is a tough one, I always keep some tension on the Jib Sheet and furler line when rolling it in so the line in the furler drum does not get too slack. While you can usually turn things by hand when everything is furled, as you proceed to unfurl the jib, the line might start binding if it was rolled in slack. The only other suggestion is to have someone watch the furler while it is being used to see if there are any other clues as to what is happening or wait until there is no wind and try unfurling the jib while you are close to the furler.
Not much to go on other than check the size of the furler line.
John Meyer
Hilbre 2135

I had a similar problem on Caprice. I overcame the problem by just slacking the jib halyard a bit and the furler works fine. i.e After the jib is unfurled I tighten up the jib halyard. When I am ready to furl the jib I just slacken her up and she rolls up easily and is ready to unfurl the jib next time.

The fact that you can turn the drum easily by hand makes this a tough question. Could the forestay be so slack that the foil has trouble turning around it when under any bending load?

When there is is pressure in the sail, the foil will form a curved shape and possibly make it tough to furl. When unfurling, the force is on the sheet which is usually a good ways up the foil and, again, make the foil bent.

Turning the drum by hand at the dock might not exhibit the same symptoms. Again, it's just a thought.