Sometimes for no reason customers get VAT reverted even without entering VAT code.
Our all buyers are from same country
I am not sure, therefore I must check you tax settings. What I think is that for this customer the tax settings of OpenCart is set to not pay tax. So not pay tax in all EU countries except the store country.
The best thing to do is you let all EU customers pay tax. When a valid vat number is given, my extension will make zero tax, you then will see an order total tax with 'Reverse charge - VAT exempt'. I don't see this, or maybe you made a language translation.