Why my customers can't receive the messages?

Hi all, i am about one month in the platform and despite the fact i answer to all of my customers no one ever texts me back. Two of them who came at my apartments told me that they never received my texts. I also tried to message them via extranet on reservation tab both by email and text and nothing. I call booking and they say it is all fine or that it is might their problem. I don't think it is coincidence that no one responds, so does anyone know what might the problem be? Thanks in advance.