Sometimes it's not returned, and/or engagement is avoided for one specific reason:
- The Thai person can't speak English (no judement intended), and they avoid engagement because they fear the foreigner will speak English to them and they can't respond and they will lose face. So avoid engagement. Not rude, just normal human behavior.
An extension of this, I speak pretty good Thai, several times I've spoken to shop (and other) people in polite Thai and contact avoided, so I ask again politely - more avoidance.
Then my Thai son (his face is more Thai than foreign) says politely, in Thai 'My dad is speaking to you in pasa Thai'.
Then a second later the penny drops and a two way conversation starts.