You can add more grains next time, but you should really dig into why your efficieny is suffering if this is indeed the case. First place I'd look is grain crush. Second conversion check. Third process. Fourth pH. Other things can include but are not limited to faulty readings, reader error, equipment error/malfunction...etc...etc...etc.
Event Horizon ~ A tribute to the miracle of fermentation.
Brew what you like. Do this, and you will find your inner brewer.