Missing purchase invoces

U CAN ADD MISSING PURCHASE FROM YOUR SIDE TO GSTR_3B 

absolutely add. it in next month 3B ...

ITC can be claimed only within one year from the date of issue of tax invoice relating to supply. In any other case, the last date to claim ITC is earlier of the two below:

Before the filing of valid return for the month of September following the end of FY to which such invoice is related, or Before the filing of the annual return

Note: As per section 44, the last date for filing of annual return is 31st December following the end of the financial year.