Less refund recieved from IT Department

I have got a refund for Rs. 7,800 from The Income Tax department by cheque for the AY 2008-2009

The details of the refund & the basis of the calculations have not been given to me

As per my calculation, I should have been refunded Rs. 28,000/-

I am suspecting that due to some error in the IT department (or maybe by me when filling the form by oversight as I may not have filled in all the columns ) my short term capital gains with STT paid  have been taken at full rates

Please advise me know how to proceed & if there is any time barring event as 31st December is approaching in case I am required to correct the return

Thanks for advising