Is this only happening with specific contacts? Maybe the problem is with those contacts.
You can try doing a network reset on the phone and see if that helps, to do that go to Settings-> Systems-> Advance-> Reset options and select reset Wi-Fi, Mobile & Bluetooth.
Also, make sure your APN settings are correct, you can find that info here.
I also found sometimes if you have a lot of messages you sometimes get issues, you can try clearing the app cache and delete some older text.
If all else fails you might have to do a factory reset.