Hi David.  The program will only display formulas that are valid for the task, so in other words if the formula isn't created using the appropriate records to calculate the total, the formula won't display in the public formulas.

If you are Tracking categories in Job Cost the formula must use fields from the category record. If you are not tracking categories the formula must use fields from the Cost Code record.

Check in File, Company Settings, JC Settings to see if you are Tracking categories, then review the records used in your formula to determine if the fields used in the forumula are from the correct record.