The reply to your query lies in your post itself. In your post you have written that you need monthly recruitment drive. But then do you need interviews every month? Is it because of expansion of your company or employee attrition. If is latter then how many persons left because they were unfit to handle the job? You need to do analysis of the candidate who were required to be removed because of their poor performance. You need to tally questions asked in the interview for such candidates and find out where your selection had gone wrong. The second analysis is about the employees who were not removed. What they told in interview and what they actually do, is there any gap? Why this gap was there that you need to study.
Unless you do this, you will never come to know where you had been going wrong.
Above, have you tried the time tested "Behavioural Interviewing"? In behavioural interviewing, candidate can never concoct answers. However, in this interviewing method, interviewer has to be trained formally. The trouble in India is that most of the interviewers assume that they have acquired these skills on their own. What is happening in your company could be the fall out of this assumption as well.
I conduct training on "Behavioural Interviewing". About five years ago, one of the members of this forum had attended my training and later uploaded his comments on the forum. You may click the following link to refer those comments:
If you wish to know the contents of this training programme then you may click here.
15th January 2015 From India, Bangalore