Lost in communication

I don't get it. Some people seem to be perfecting the skill of selective listening.
For weeks, I had been going on about a particular task. I don't even expect people to think for themselves. But continuously spelling things out, and things still not getting done is extremely frustrating. And causes a lot of problems, too, of course.

I am aware of the concept of sender and receiver - so where is it going wrong? And how can I encode my message in a way that it gets properly understood by the receiver? Or is the message actually being received correctly and just ignored?

1.5 weeks until holidays - I can't wait!