Dear Swati,
So many have contributed with very good suggestions, especially Malik ji and Sajid Ansari ji.
I would like to add a few important points:
(i) "ABSCONDING" is not the right term to use; see the meaning in the dictionary, and you shall know it reflects badly on your company's culture. Hence, I am not surprised that potential candidates don't join your company.
(ii) Please review your selection process and the panel.
(iii) It is possible (highly probable) that your company is chasing too good candidates without offering much in terms of emoluments or a great working environment. Maybe the company's brand equity as an employer is not substantial. (Else, why would a person not join?)
(iv) Remember the oft-quoted cliche - "You can bring a horse to water, but you can't make him drink." Your top management needs to ponder over it.
(v) You should always maintain a panel of candidates in order of their performance and keep in touch with at least two. It is easy to call them up, thank them for their participation, and ask if they are still interested in your company. That way, you have ready replacements. But you will have to shed your ego to achieve this; as generally, HR is not bothered about candidates down the line.
(vi) One should never mention any financial data on offer letters, appointment advice, offer of appointment, or letters of intent. All the above are several nomenclatures of documents that are issued after the selection process and before the joining of candidates. You can have a paragraph stating that "...the detailed terms and conditions, including emoluments and other perquisites, will be issued along with the appointment letter upon joining within the stipulated period."
(vii) Also, add, "This offer shall automatically expire after the stipulated date of joining mentioned herein."
(viii) In case you reimburse traveling expenses, there is a considerable financial loss to the company. Hence, in one paragraph, you must also mention, "In case you are not able to join within the stipulated date, the expenses reimbursed shall be recovered from you along with any other expenses that the company might have incurred during the course of the selection process. Suitable help, as deemed fit, shall be taken from your existing employer in this connection."
Hope the situation will improve.
Regards.