The area in which you can successfully dig up the item is incredibly tiny. The only time I can successfully dig one up is if I actively /follow a goblin digger. When they do the animation, you are right on top of the goblin, and just /dig right at that spot.
If you get it just at random, it really isn't worth spending the greens trying to dig it up, best to keep moving.
War Whm Brd Pld Drk
A good player is one who understands the potential and the limits of his job abilities, and works conscientiously to use those abilities to the maximum benefit of the party dynamic.