I took the time to email someone about a specific problem and didn't hear back - how long does it take to hit reply and type "Thx"? The effects are corrosive: why should I bother to reply next time?
That's not an excuse. It's a total failure that I forget to say thank you properly whenever it's deserved (and even when it's not). It's something I am working hard to fix, but I still frequently mess up.