Some text messages not coming in to my iPhone

Lately I've found some text messages being sent to me are not coming in to my iPhone. The senders can get my texts, but I'm not getting theirs. I've stood next to them as they send to me and nothing comes in. When I send to them they receive immediately. The texts to me aren't late - they aren't coming in at all.

And this is only with a few certain people, not all. With others, sending and receiving works fine.

Is there some setting that may have blocked these folks?