A "dry spell" originally refers to a period of time when there is no rain. Example: "We've been having quite a dry spell lately; it hasn't rained in over two weeks." Likewise, to have a "dry spell" is to go through a period of time when one is not getting something that one wants. Example: "Sam is a great salesman, though lately he's been having a bit of a dry spell." Even the best professional atheletes have "dry spells"; periods of time when they don't play as well as they usually do.