Welcome to the World of Recruitment!
Many good points have been made here, and some seem rather out of place to me.
Factors Influencing Recruitment Success
IMHO, it depends upon:
a) Your Company in competition with that of the candidate's. Consider factors such as brand, turnover, perceived employee friendliness, and salary structure. If your company is better, you have a far stronger chance.
b) Your Screening Process... If you have been thorough and the candidate can see a clear sync between what they are doing and your job description, along with a stretch in job responsibilities/career, they will be more interested. If they perceive your efforts as a random "call all and let us see who turns up" approach, you will experience a high dropout rate. This happens more often than any HR professional cares to acknowledge, and candidates are smart enough to quickly catch on to this fact. Repeatedly doing this can tarnish your company's reputation.
c) Duration of the Open Position - The longer the position has been open, the more the "available pool" knows about it and loses interest accordingly.
d) Employee Feedback - What are your present/past employees saying about your company? This is difficult to gauge, but with the rise of "networks," it is an easy source for any candidate to turn to.
e) Interview Scheduling - How difficult/easy are you making it for the candidate to attend? Some companies schedule interviews on Sundays and close the process that day itself for junior levels, with at most two more rounds for middle/senior levels. Where do you rate?
Despite all these factors, there will be dropouts... emergencies will happen, urgent tasks will arise... You can only check with the candidate about an hour before to have a clear idea and act accordingly.
And, if possible, close the position on the same day, even if only two or three candidates out of ten turn up. (In fact, I am rather surprised at that number; it seems rather high to me even for junior-level engineering openings.) It is a difficult call, I know, since hiring managers are always interested in seeing "more" and blaming HR for not achieving their targets. This will depend on top management's attitude and your relationship with the hiring managers. Get a reputation that your company is serious about filling positions fast, and you will find many things falling into place.
Just my thoughts....