I use 4mm rope with a dyneema core. At last years Dinghy Show a Marlow rope representative said that 2mm dyneema was plenty strong enough and said that one could strip the outer polyester layer from a 3 or 4mm rope (dyneema core) and just leave the outer covering near where the cleat is.If you use a Clamcleat you may have worn the grooves over time. In this case replace the Clamcleat.