Chicken Fettucini Alfredo

I had made this once before and was excited to be able to cook it for my parents this time. However, something went amiss when making the alfredo. For some reason the parmesan cheese didn't melt into the sauce properly. I don't know whether it was a too low of temperature, or adding the cheese too fast, or something with the milk, or perhaps due to the fact the cheese had expired a few months before. In anycase, the taste was fine in the end, but it looked a little strange. I don't know why things always have to go wrong when I'm cooking for someone else...