I think that you maybe should show some respect, as Thailand have many dialects which makes it even a little bit hard for themselves to understand it all. Also, some people do not speak very clear, which many times have a connection with no schooling or very bad quality schooling. Your British humour, will only work for you, as they will see it as an insult at best which is bad behavior from you. I suggest that you tone it down and use English if Thai does not work for you, as that seemed to work fine.
As a side note, I also speak, read and write Thai fluently. Personally, I never had a problem you are describing.