Formulas for CRM Enterprise- Limit on number of fields

Formulas for CRM Enterprise- Limit on number of fields

Hello,

Trying to create formulas in our Deals module. we have a limit of 15 and so far we have used only 13. We have currency and date and time fields still available and no formulas in our unused fields. 

Why is it not allowing us to use the two formula fields we should have left?