How are you checking that it is calculating? Press F9 to make sure. Are you sure that you are changing the right cells? Many models have a hidden inputs sheet which all the output sheets take their values from. Is there some horrible macro that you need to run?
That's all I can think of without actually seeing the spreadsheet.
if the model is very big, it's probably set to not automatically update to avoid slowing it down (i am forgetting how to do it, but i know there is a way). you need to refresh it manually, by pressing f9 i think.