My Thai students aren't fluent for two reasons:

Reason 1--Disagreement over the definition of 'fluent'.
Thai parents have an idea of what level of English fluency is acceptable for their kids at different ages. We farang have another idea. Right now there seems to be quite a schism between 'our' and 'their' definition.

Reason 2--Students' underestimation of the amount of effort required to become fluent.
While we were growing up back in the (English) West, we were surrounded, even bombarded, with English language. It was EASY for us to learn English. We couldn't help but to learn the stuff, and we learned it with minimal effort. This explains why native speakers can speak English perfectly without really knowing grammar--it was TOO easy. Thai students have no such advantage. With far fewer native speakers around, and the lack of other sources of English, they have to exert MUCH more effort to achieve the same level of fluency. And from what I've seen so far in Thailand, students here are not
willing to exert the effort to become fluent. They won't even come close.