Generally shortages crop up everywhere from time to time. It all depends on retirements/resignations, a change in aircraft numbers at a particular base. Some bases are more popular than others so people will be bidding to leave their base to move to another.
It seems to me that it's the Scottish bases (GLA/EDI) which are short of crew at the moment. But that's just my take on it, chaps from other bases may tell you differently.