iMessages not working

I had trouble receiving and sending iMessages on my iPhone 5 in 9/14. I upgraded to an iPhone 6 in 1/15 and still have issues. My girlfriend, who has an iPhone, will send me messages and I don't get them. It doesn't happen all the time and seems to be random. Occasionally, other iPhone users don't receive mine. I have been at my local Apple Store several times since January and they don't know what is wrong. I tried everything they suggested. I've just been dealing with it for several months now. Is there anything I can do? Is this a Verizon issue or an apple issue?