You didn't say what kind of main sheet system you were using.



Some of the older Seaway blocks had cleats that were difficult to engage under a load. If you're already block to block, or holding all your arm can stand, you'll have to back off quite a bit, before cleating. You might be able to replace just the cleats with the newer harken cam-matic or going down to a smaller 3/8 size sheet if you're using 7/16.