Tax line is not calculated when entering the tax classification code on the invoice item line

Hello, when entering a specific tax classification code on the invoice line and trying to "calculate tax" or run Invoices Validation nothing happens, the tax line is not calculated.Although, when entering the invoice line *without indicating the tax classification code( the countrye`s VAt rate is getting calculated and afterwards we can go at tax details and update it manually with the desired VAT code. Any suggestions ? Thank you for your help, Florina