Had a nice chat with a local HR manager. She complained that a lot of workers simply don't show up after signing their contracts.
We did a little analysis ( Systems thinkingto be precise) and tada we found a culprit: "We regret that only short-listed candidates will be notified", the standard sentence under each (print and on-line) job-ad.
You leave Job-Seekers in the dark about the job. So they need to guess if the position is still open or they can move on. So if you offer the B-Job and the company with the A-job is slower than you (which they often are, since more people apply), you will always loose out.
Solution: Acknowledge the receipt of an application and send out status updates including when the seeker is out (short-listing, position closed). Doesn't cost much (just an email) and tackles the problem (in the long run). It is also a good idea to ask for permission to store the profile, so your HR department can build a little head hunting database of their own.