By law, sales outside the EU and certain other transactions are tax-exempt if the export has been confirmed by customs (tax-administration sub-department). If you meet the requirements and ask for a VAT refund the store should refund your VAT.
Unfortunately, some stores don't cooperate. Either because they don't understand the law, the administrative procedures involved or because they have a contract with another tax-free company. These are not valid reasons to reject your claim.
If you meet all requirements and the store does not want to cooperate you can continue with the steps below: