Motorola Moto G — Phone App Acting Up

Hi there. I don't know if this is a problem with the phone, the app it's self, or my phone service. The problem I am having is that whenever someone calls my phone, the usual "popup" that appears on the screen doesn't always appear, so I cannot answer the phone. On the odd occasion that the popup appears I can sometimes see the popup that has a green (accept) or red (decline) button. On other rare occasions I'll see an old style popup on the screen. I have uninstalled the phone pp to its' factory setting, and kept that for a while, but that hasn't helped at all, I still get the same problems. Another occasion when the phone was ringing, I saw a tiny phone icon in the top left of the screen, and was able to "drag down" and see the green/red popup. But then the next time I saw this tiny icon and dragged it down, nothing showed. I am now at a loss as to what I can now do to resolve this problem, and it's getting to be rather frustrating since I sometimes miss important calls. I will say that thankfully folk's will leave a message, but to get them I usually have to reboot the phone.
Any help/suggestions regarding to this problem, will be very much appreciated.
Dave.