Don't see messages from one specific sender any longer
I am guessing the original poster does not need this answer any longer, but I found the answer after having this same problem for months, so perhaps posting here will help others resolve it more quickly. Open your text stream to the sender for whom you are not getting alerts. In the upper right hand corner, you will see a "Details" Link. Click that, and then look for the Do Not Disturb Switch. If that is green (switched on), then you will not receive alerts from that user. The killer thing about this is that you are most likely to accidentally switch this on for someone whom you text often. It is no fun when the only person for whom you don't receive text notifications is the person you most need to hear from, like perhaps a spouse . . .. Glad I got this corrected.