Antenna question

Why are my ABC, CBS, and PBS stations coming in clear and strong and my Fox and NBC stations buffering at times? They are close together and in the green too. I am using a GE UltraPro Edge HD amplified antenna pointing north above my windows. I'm running it through my Airtv Anywhere that I just got running again with a new power supply. We are trying to see if we can record locals first before we drop YTYV. Thanks in advance.