Recruiters - What are the issues with finding workers?

I read many posts from recruiters that talk about not being able to find workers. There is often little detail other than saying people don't show up for interviews. What happens then? Do you go back to your pool of resumes (that job seekers think you are keeping) and schedule new interviews? There must be enough people who are qualified before assuming that no-one wants to work. Lots of people both want and need to work

I'm talking mostly about unskilled, entry level, customer service jobs that are simple enough for most people to do.

What can job seekers do to match what recruiters are looking for?