Check the tape. Did the stock get taken by someone else? Then maybe you were a hair slower than someone else. It could also be that you may be getting slightly delayed data to your execution platform. The stock that was offered also could have been cancelled before your order got to the offer. I've received a lot of partial fills on orders like this because someone else was faster to the stock.
Quote from bonds:
can someone explain why my market orders on hybrid are no better (and many times worse) than they were before?
i have on several occassions seen over 5k on the offer and i will market buy 5k immediately and get an average price 2-4 cents above the offer... how can this happen with the hybrid?