Whatsapp msessaging problems

The other day I was using Whatsapp and talking to a friend (1). Later that evening I sent a msg to another friend (2) without issue, however, I noticed later that this msg had been sent to friend (1) too. I went into the conversation for friend (1) and discovered that the previous text msgs were now no longer visible and rather strangely when I spoke to that person they informed me that they had never received the "random" msg intended for friend (2). In addition to this, I have experienced other problems with Whatsapp when texting friend (1), specifically that I will receive a whatsapp msg from them, part of the msg will flaash up on my screen but when I enter Whatsapp to read the msg, there is no msg. Ony ideas as to what has gone wrong? I even did a factory reset just incase my phone had been compromised..
Thanks Trevor