Missing values when extracting data from Facebook Fanpages

I extract a lot of data from fanpages 2 times per month, 1st half and 2nd half, for different fanpages on Facebook.

I noticed recently that sometimes lets say i try to extract 1-15th, some of the data of 1-4th or even 5th will be missing and ill need to run the queries by segmented days.

Why is data missing?