The system uses two lines. It sounds like you may be trying to cleat off with the smaller diameter line. What Hobie calls the "halyard line" is a 1/12" diameter batten tie line. This long thin line is only used for raising the sail. Once you have the sail up, you remove the halyard line and do the final tensioning and cleating using a shorter, larger diameter "tensioning" line. I use a 4mm diameter line for the tensioner, no issues with slipping in the cleat. I also tie off the excess line around the forestay to make sure it's held in the cleat securely.