It is hard to say what can be the cause, but you should monitor your mining activity for say a week, and make note of how many people (terrabytes) are mining when you are, shown by the netdiff value in Blago's miner. That will give you an indication of the current difficulty and if your mining income varies a lot from day to day compared to your average.
Also, luck plays a part, and sometimes it takes a while to get the full payout from the pool.