Sum stops working after

Hi, i'm fairly inexperienced with using advanced formulae with excel.
I've been deemed the task to maintain our sales spreadsheet, however i've found that as i import more data, our Sum has stopped calculating when i do auto filters? I'm not talking about manually selecting cells and summing them up, i'm talking about determining how many rows there are containing selected infomation when i auto filter. (Eg: 57 of 2002 records found)
When i first imported some data, and right clicked down the bottom where it says Ready, and changed it to Sum, it worked fine, i've tried to do conditional sum changes, but i can't seem to make them work.
Now every time i get past about 40 records/entries, it the sum will say calculating, and then just simply say Ready.
I also have a differant spreadsheet, which has over 2000 records, which is still summing fine?
I've checked in options to make sure it is still saying Automatic, and cannot figure out what setting i need to change to allow it to sum larger amounts?
Any help would be greatly appreciated!