It looks like you have a slipped half-hitch on the rope.

Is the half-hitch there because you experienced slippage??

Or is it there because you just feel safer with it??

If because of slippage, would increasing the length of the buried portion increase the holding power enough to make the half-hitch un-necessary?

What is the length of your buried portion?

For the Whoopie Sling, Samson Ropes recommends that the buried portion be equal to 3 1/2 fids, so for your 1/4" diameter rope, they recommend the buried portion be 18 3/8". It is hard to tell from your pictures the length of the buried portion.