I have created a custom field that uses the formula "amount - setup_fee_c" and for some reason it only works some of the time... As in, half of the opportunities will show the custom field correctly based on the above formula, and the rest will just be blank. Any idea why this is the case?

I have created a custom field that uses the formula "amount - setup_fee_c" and for some reason it only works some of the time... As in, half of the opportunities will show the custom field correctly based on the above formula, and the rest will just be blank. Any idea why this is the case?