Link Clicks Decreasing?

Hi There. We have bitly links on all of our email signatures to different pages on our website and google/facebook reviews etc. The strange thing is that the number of clicks sometimes is a lot less than the previous month when I do my monthly reporting. I can't see how this is possible as you can't do negative clicks on the links. The links were always increasing, as they should, for years but after the last few months a lot of the links are showing a decrease in actual number of clicks. Can anyone explain why this is happening? Any advice much appreciated.