Galaxy S6 :: Why Do Continually Not Receive Texts From One Person

So, I've had this phone for almost 3 months now. I'm barely able to text one of the few people that I talk to. Most of the time, they do not receive my texts until a week later, sometimes longer. I used to have the galaxy s5 with sprint and this happened with them too. I now have t-mobile and i have talked to them several times about this. After a lot of persistence, they put in a trouble ticket for me and that corrected it for about a week, then it started up again. Now they closed it and are telling me that it could be sprint's fault. I don't know what could be. I know that at one point I was accidentally blocked by the person, but it seemed to start after I ported my number over to a new carrier. I just want an end to this. I'm tired of contacting t-mobile every week.