I fully agree with what other members have suggested you. I need to ask you , are the candidates not joining after receiving the offer from you? If yes, then (my quick thoughts, though not number wise)
** You need to have a quick look at your interview process, right from the moment they enter the office till they leave your premises,most important is to make them feel welcome. - Are you noting and taking full job history like past records etc or are you offering jobs to those who frequently jump from one to another- job hoppers.
** Did you highlight all positives like the post - career graph from there on? benefits of working with particular company? in particular profile? Are there any particular benefits that could boast about like group mediclaim etc? Same thoughts as Mr. Pavan has pointed out.
** Compensation part: I might offer you a very high CTC but when you see actually your take home would be too less, since you have provided vague information on this I will leave the judgement on you only. Whether you present a lucrative offer or not?Is it on par with what your competitors are offering.
** are you asking them to submit copy of their resignation letter within stipulated time period (which starts from the day when you release an offer)
**Stay connected with the candidate. they might be dabbling with two or more offers. or could be fishing for offer to negotiate at the current place.Like when our employee had to joing us from one reputed MNC, we were in touch with him knowing about his exit interview, last formalities like handing over,his shifting ( as he was working in Mumbai and we are in Gujarat)
Though once when the candidate refuses to join you, you cannot expect them to tell you the true reason but still you could take a chance and ask them very gently about the reason. Do not probe much.
I am sure your consultant will be having pool of eligible candidates. And let us know what reason has s/he offered you.
Best Wishes