Ipad not receiving messages that are received on my phone from other iPhone users

Hi all. I just noticed that since early May, my iPad no longer receives imessages when others are sending them from their iPhone. If someone just has an iPad and sends a message via my Apple ID, then it is received on both my iPad and iPhone immediately, but my iPad is no longer receiving the iMessages that people who have iPhones are sending me. It comes up on my iPhone as they are sending it from their phone to my phone number, but not my iPad. I have checked the settings and they have not changed, the "send and receive" still shows both my phone number and Apple ID. Is it anything to do with the fact that it is an ipad4?