Very interesting discussion.
I have a 5Kg Bruce and around 5 to 7 metres of chain. I have never had the anchor slip until the last 2 trips to the island.
Found the advice here was the answer.
I was dropping the anchor to the bottom and then letting the boat run back whilst letting the anchor rope run out as boat moved with the tide. The boat then swung around as the anchor "bit in" but then started dragging. ( 200 metres of rope out by now, so that was not the issue ! )
What I found is if I pay out some rope as the boat moves in the tide and hold the rope I can then feel the anchor dig in and then let more rope out as the boat moves back ( this is easily done side anchoring ).
Even so, last time on the spoils I was anchored for 2 to 3 hours and then the damn thing started slipping. heavier chain is probably the answer.