Appearing to be well off can also mean well in debt. They may have appreciated some help back.
Parents do not wai their children back no matter how old they are. By extension they may consider you as one of their children. You should only expect a verbal response when you wai them. As a side note, when a child wais you don't wai them back, just a verbal response. There are also different nuances of respect when waiing. Such as how high your hands are and if you nod your head or not.
If it was the first time you met them they may be unsure of how to act with a farang. But may warm up upon getting to know you better.
Most Thai's give foreigners a lot of leeway because they know they are not familiar with Thai customs. But don't try and push your customs on them either. They just aren't used to it.