The most challenge is the big demand of IT people (in general) and Project Managers professionals (in particular), most of them are just looking for a salary increase without caring either the kind of company or the style of project, that's why, they are not committed to the company in spite if they could offer good benefits, a career path or a stablished job.
At least in Latin American people, they don’t have long term vision, so those people are not able to realize the benefits that could get if they worked one year or more in the same company; sometimes they prefer to jump to another company that offers more net salary, without reviewing if they could end up earning less money because of the less benefits that the new company has.
That's why it's important to make a better selection of them, searching his short, mid and long term goals that the candidates has, and the work path that they had had, before to hire to the enterprise.