Cell formatting disappears

Are you entering the numbers directly into the cells or are you copying and pasting them into the cell?

The format should not change if the numbers are directly entered, but if you are copying and pasting numbers from another source, the choose the Paste Special > Values option to preserve formatting.

If this doesn't help, perhaps you can post the problem workbook? Does this problem manifest itself in a fresh workbook?

I am putting the numbers directly into the cells. I dont know how to paste the workbook im afraid, I will try the paste special.