This is a question that I am asked often at the moment, and one that has myriad of answers depending on which sector the recruiting agency is in. Traditionally there have been certain sectors of PR including healthcare, technology and financial where there are simply not enough candidates of sufficient calibre needed to perform the roles. In recent times this has grown to include some specialist consumer agencies too. This article in PR Week examines some of the difficulties that agencies are facing and ways in which they can overcome them.
http://www.prweek.com/uk/features/1105582/recruitment-desperately-seeking-account-managers/