Dragging the fill handle leaves cells blank

I have a financial workbook I have been working with and I'm just starting to come across an issue where sometimes if I drag the fill handle to copy a basic SUM formula down a column it will instead erase the contents of the cell and leave it blank. Sometimes the cell will return 0, but not have the formula in the cell, just 0. This seems to happen in specific places of the workbook, but not all of it. Any idea what is causing this?

I have tried pressing control and selecting copy cells when I drag the fill handle but it didn't change anything.

Sorry no, not the cell containing the SUM formula. The afflicted cells are those I am dragging down to. Some number appears (or doesn't) and there is no formula attached to it.

When the cell returns a number with no formula it is not always 0. It returns various numbers in different parts of the workbook.