This may be away too simplistic as so many riggers etc have looked at the problem.but here goes
I have solved MANY similar problems with above deck furlers, all of them having been fitted by professional riggers. ( no experience of under deck furler).
The angle of the furling line coming off the drum was wrong. So tiny it’s not obvious to the naked eye. Solved by experimenting with moving the nearest lead in block to the furler drum up and down and backwards and forwards tiny distances . Requires patience as 2cms difference has proved crucial. There are recommended angles and distances for the furling line off the drum quoted in the original fitting instructions .
Best of luck.