The problem is, it depends on where the Thai person learnt to speak English if they learn from their friends then that's the English they understand, not your English and yes many can speak some English but don't understand English hence the blank stare, then you have all the different English accents, my English accent is still quite strong so I have to tone it down if I'm speaking to a Thai or the staff at 7/11, but I can speak Thai anyway,
Don't forget all the other foreigners like Germans/French/Italians/Russians all speaking their versions of English not easy for them, I remember sitting at a bar one night got these Russians trying to order cocktails and drinks and the girl was not understanding them, the guy remarked why don't they speak English, I chipped in and said they do speak English but you are speaking English with a Russian accent that's what they don't understand, ok tell me what you want and I will translate it for you,