If someone doesn't understand what you say...?

I used to encounter this problem when dealing with customers. But I just think that there's a communication barrier because of different nationality. so what i do is just to repeat what i have said making saying it slowly and asking the person im talking to if they understand what I said and if not then I will make it much more slower and changing the word to a more common used or easier to understand

Sorry, I misunderstood the question. There is another aspect of wavelenght. Sometimes people cannot see it from another point of view because they cannot understand. It is not that you are devious or they are ignorant. It is only that you don't share a wavelenght and that people do have different perceptions.

If the 'someone' you mention is a normal intelligent person and your question is regarding some personal and not technical issue[say for example you ask a person to understand technical jargon and he does not know a thing about it] then it means that the person does not want to understand. It does not suit them to understand your point of view, moreso when the person is required to act as a follow-up measure. You see what I mean? So, we should think that the person pretends not to understand because'devious' may be too strong an adjective in some contexts where the intensity of the issue spoken is not great and it is only a trivial issue. I would say that it is deliberate.