DCAstudent hit it right on the nose,,,
"As for the Dell example cited above: it seems that if a merchant in any way alters your order, then you don't get the miles because the entire purchase has to be completed through the eshopping portal. By involving Dell, eshopping probably lost its commission"
Many of the folks complaining about not getting miles ignored the many warnings about NOT entering a discount code or in any other way altering the order.
And as predicted these folks are now wondering where there miles are.