I plated mine just like you did (well, I didn't have a handy spacer grid), 4 in. apart. However, when I look at yours in the picture you linked to, mine looked like they were closer than that...so perhaps your 4 in. was farther than mine
However, I'm inclined to agree with you that it's probably the sandy soil.
I know how you feel about the blueberry bushes. I had one and the deer ate it in the summer and winter, so I moved it to the garden....when a loose dog came by and munched on it (I have no idea why).
This year, the animals left it alone, but I was hoeing and accidentally knocked off a branch
Still, I did mange to get a blueberry or 2 off of it