I fully agree with what other members have suggested. I need to ask you, are the candidates not joining after receiving the offer from you? If yes, then here are my quick thoughts:
Interview Process Evaluation
You need to have a quick look at your interview process, right from the moment candidates enter the office until they leave your premises. It is most important 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?
Highlighting Positives
Did you highlight all positives, like the post-career graph from there on? What are the benefits of working with your company or in a particular profile? Are there any particular benefits that you could boast about, like group mediclaim, etc.? These are the same thoughts as Mr. Pavan has pointed out.
Compensation Considerations
Regarding compensation, you might offer a very high CTC, but when candidates see their actual take-home pay, it might be too low. Since you have provided vague information on this, I will leave the judgment to you. Whether you present a lucrative offer or not, is it on par with what your competitors are offering?
Resignation Letter Submission
Are you asking candidates to submit a copy of their resignation letter within a stipulated time period, which starts from the day you release an offer?
Staying Connected
Stay connected with the candidate. They might be considering two or more offers or could be using an offer to negotiate at their current place. For example, when our employee had to join us from a reputed MNC, we stayed in touch with him, knowing about his exit interview, last formalities like handing over, and his relocation (as he was working in Mumbai and we are in Gujarat).
Though once a candidate refuses to join you, you cannot expect them to tell you the true reason, you could still take a chance and ask them very gently about the reason. Do not probe much.
I am sure your consultant will have a pool of eligible candidates. Let us know what reason they have offered you.
Best Wishes