Have you looked at the clam cleat teeth ?
Are they worn ?
Because, I know some that goes out riding, without dipping the cleat in the water (and it has to be EVERY time you go out, if a sandy beach) - and this can destroy a good cleat in a few sessions.
Sand will EAT the teeth really fast so they are worth nothing, and the line will slip
I've NEVER had a cleat that got worn, not even after many years, since I've just dipped my cleat in the water whenever I go out
Many years ago, when cleats started being used, we destroyed the cleats really really fast, because we did not rinse for sand
And no, it was not the cleat quality that lacked, as the very same cleats, when new, hold up "forever" when just taking care, as we learned the hard way at first...
Takes a few seconds only, to dip the cleat in the water (just fly the kite down low and easy to do when going out), and you are good to go.
I've never had a depower rope that got "slippery" - only worn so it actually sticks even better in the cleat (but eventually has to be changed if frayed too much), so I think it could be your cleat that is "wasted" and useless ?
Take a close look, maybe with a magnifier glass, and compare to a new one if in doubt - is my advice