digg'ing comments

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I noticed that everytime I dugg a comment, it showed as having +1 digg for a little while, then afterwards has zero diggs. Is there something that disables friends from digging each others comments? It's the only thing I can think of because all the people were folks I had added to my friends list. Otherwise, I don't understand why the comments would "lose" their +1 digg so soon. Can anyone explain this?