I am not receiving messages from some people on iPhone

Since returning from o/s I am not receiving messages from some people. I have iMessage turned off as I share my apple ID with my 2 kids (& Facetime) and our messages will share. If someone turns off their iMessage I will 100% get a message from them. But sometimes I do receive a message from someone with iMessage on. I have reset my Network settings 3 times (this only changes who I can receive messages from too, weird!) and turned phone on and off. Not sure what else to do.

[Re-Titled by Moderator]