Short answer first: When you add or make changes to a formula of a pre-existing field, you will need to click on the "Advanced Setting" link under the formula and choose to apply this formula to all saved records.
The reason behind this is that Ragic is actually a database application, not just a flat spreadsheet file. So every record you have is saved in a database as a record with revision history, metadata, approval flow, comment threads, etc. Therefore, changes will need to be applied explicitly so that your database records are not changed unknowingly.
Another reason is that the formulas in Ragic work as a type of default value, meaning that after the value is being calculated, you can still change it afterwards and save your database record. This enables you to for example use formula to calculate a default price for a sales order, or a estimated shipping date, and change the value of the result to make manual adjustments. Ragic will keep a history of every edit you make, which is visible through each entry's information panel.