Test the Questionnaire on Colleagues and Revise It

(Yes, you are right. This is a usability test of the screener!) If it is a phone questionnaire, use the phone. You would be surprised, but it makes a difference. If it is an email or web-based questionnaire, test it that way. Observe as a colleague tries it out by answering the questions to see what types of answers you get. If they are not what you are expecting, you must revise the questions, so it is clear that future respondents will understand what you want to find out and so you will be able to determine easily whether they qualify for the
study.
Regardless of whom you use to help you acquire participants, control the process as much as possible by being as specific as possible about your needs, anticipating miscommunications, and writing down the exact words you want the procurers to use. It is your study, they are your results, and the whole affair is seriously compromised when the wrong people show up.