Probably the biggest cause of a jammed furling line is letting it free run during the unfurl, especially in a breeze. This pulls the loops into the furler at a high rate of knots , and the loose loops can overlap and override each other.
It's really important to keep some tension on the furling line as you pull out the headsail with the sheet. Take a turn around a winch or a cleat on the larger sails that can overpower someone holding the furling line, especially when a good breeze grabs the sail.
Prevention's worth a.....