Why cant i text certain people??

ill send the certain text messages, but theyll never recieve it, even when my phone says its been sent, and whenever they send me text messages ill never recieve it. so i went to the t-mobile store and they gave me some bs saying its not my phone or networks its ALL the other peoples phones. so i got a new phone to see if it was my phone..it wasnt, and my friend happened to get a new phone [who was one of the people i could text] and even tho we both had new phones we still could get eachother texts, so wtf is going�on??? how do i fix this??/ out of the 70 people in my contacts i can only text like 30 of them. and its not an issue with the area code, cause it says the message was sent.