iMessage and Text failure just to one contact

I am not able to send iMessages or normal texts just to the one contact.

Sometimes their sending as iMassage and sometimes as texts but most often appearing failure. Same with receiving messages from that person their stock and are not delivered to me. Also that person has problem to send messages to me. Today even normal text messages failed.

We have checked everything, restarted our phones few times and turned on and off iMessage function. Have to add that we both have internet connection and network connection. Never had problem with network as using same for 2 years. I have new iphone 6 from 3 weeks and the problem first time appeared 5 days ago.