RE: Whitetail Institute "Extreme"
I'd be happy with 6.4 for most clovers. I'd bet that the sandy soil dries up easily and the clovers cannot develop enough to prosper- or you are deficient in P or K.
Extreme is a different type of plot than I like. I do not have 1st hand experience - but I see how it can work. If clovers fail - then I too would trie something else - but with a sandy soil - I'd probably lean towards fall planted Oats, Rye or Winter wheat - since its easy to disk up.
Maybe try a fall planted cereal grain & a good white clover. Maybe getting the clover estasblished before spring rains would be the ticket.
FH