Debit/credit confusion

So far I have been using my Finnish bank card for paying but it makes shop assistants always confused. Shop assistants do not expect that with my card you can pay with credit or debit so they start to call their collagues, pull off my card or something else. I just try to advice them to choose debit option but they have difficulties to understand me. Still I have been always able to pay. I have just received Polish bank so I can start to use it soon.