There could be couple of reasons for which job candidates could be declining the job offer at the last minute. One is the wrong selection. Assessing suitability apart, you are required to check how strong need is. If the need is strong then obviously candidates will switch.
The second reason is your company's status of start up. It is a dream of every job candidate to work in a branded company. If not in branded company, then they could go for at least mid-segment companies. Start ups are at the last rung of their choice.
The third reason could be work environment. Once the job candidate steps inside the company premises, does he/she get a pleasing environment? I do not mean that you should have very hi-fi decor or ambience. But then location of the company, road that leads to the company and actual interiors of the company also matter.
The next reason could be the existing staff. Do they have professional approach towards their would be colleague? How are their communication skills, body language etc?
You need to wade through these starting troubles. These are common across the industries. Notwithstanding the troubles, grow your company into the brand and tide over these problems.
All the best!
Apart from the comments given by other members, I would say its a Pandora Box. There are multiple reasons for a candidate to reject an offer. The main challenge I see is that, any candidate after getting his/her first offer will be very confident and his/her performance in the other interviews would drastically pace up. Also being a human basically they tend to take chances, by attending multiple interviews simultaneously.
Nowadays I see many candidates wanted to join Startups. The main 2 reason I see is the salary package and the culture of the organization. Wherein if you ask a candidate to name 20 startups, how many would be able to tell, apart from Flip-kart, Mu-Sigma, Snap Deal, Myntra, etc. Bangalore, India has maximum no of startups (More than 17,000) in the world followed by New York, US.
It's a very common issue, especially in IT start-ups. Below are the few points that might need consideration:
A regular follow-ups and open interaction will help gauge the prospective candidates intention of joining.
Its very crucial to know their current status on employement: Resigned already, how many days pending in notice period, reason for job change, etc.
Candidates job change priority: is the candidate looking for Monitory gains/ job security/ perks and benefits etc.
Also, something as trivial as their mode of commute to work plays a huge deciding factor.
One can also chalk out the growth plan for the prospective candidates so they have a clear picture of their presence in the company.
Hope it helps. good luck