I had the same trouble with the furler not holding a partially furled sail. My problem was a slightly bent clutch pin (which effectively shortened it) and a pin plate with elongated and ramped holes - I imagine due to those before me ignoring the problem.
I had a new clutch pin fabricated about 2mm longer and that solved the problem. I had considered reinforcing the pin plate but as long as the pin goes all the way through it wasn't necessary. There is room for the extra 2mm below the pin plate.