I have a close friend like this. We've always had have great two way conversations.
For the last 6 months or so, this friend will send messages asking for my opinion on something or asking questions and when I reply they drop the conversation. Then a few weeks later same thing happens and again once I reply conversation dropped.
I'm at the point where I want to either stop replying or ask why do you keep doing this? But that seems a bit dramatic doesn't it - would you bother saying anything?