European Union tax law states that a 0% VAT rate applies to exported goods. NATO employees and persons working for exempt EU institutions benefit from 0% VAT rates as well. If you meet all the requirements and export has been confirmed by customs, the store should refund the VAT which was paid when you purchased the goods.
Unfortunately, some stores don't cooperate. Either because they don't understand the regulation, the administrative procedures involved or because they have an exclusive contract with a different tax-free company. These reasons are legally invalid for rejection of your claim.
If a store doesn't want to cooperate, there are 2 things you can do: