Your bow might have worms.
Years ago, we had a friend tell us bows were nothing but a bag of worms. He was referring to that fact that, unlike a gun, there are many things that can go wrong with a bow that would cause us to miss.
The good news is that bow quality has improved greatly over the years. Broken limbs and cracked risers are (mostly) a thing of the past. Some of the biggest breakthroughs have been in string/cable quality. With good strings/cables, we have very little stretch, which helps our equipment achieve unparalleled consistency.
However, even with all these improvements, we still encounter problems from time to time. The problem that plagues us most is the peep sight and/or nocking loop changing location. Even small changes to either will result in dramatic changes in where our arrows hit. For the best consistency, we need our nocking loop and peep to stay put once set. So, we tie in our peep and really tighten our nocking loop (or tie in a nock set). The problem is, it doesn't always work.
This issue really came to light one time at a tournament. It was a hot day. One of us was shooting great, but then started shooting lower and lower as the tournament progressed. We couldn't find anything wrong with his equipment. So, we figured it must be something he was doing.
When he got home, he put his bow on the shooting machine to see if it was him or the bow. He found the bow was indeed hitting very low. Upon further examination, he saw some serving separation below his nocking loop. In the hot weather, the wax on his string became a lubricant, and his serving had compressed and slid up his string, causing the low shots.
After seeing that, we started to monitor our nocking loop location, and anytime we started shooting low we would take a measurement using a 6-inch ruler we carried with us. We would measure from the top of our peep sight hole to the inside of our nocking loop. We were very surprised to find our serving and loop were moving more than we thought. In the past, we would just make sight corrections to account for the migration. But knowing what was moving gave us an opportunity to fix the problem, not just adjust for it.
What we typically find is that our center serving is not sliding up the string; rather, it is compressing. The nocking loop attaches to an area where the string is forming an acute angle, causing the serving to compress and expand over and over. As a result, the nocking loop slowly moves up toward the peep sight.
Once we started keeping track of this measurement, we found our nocking loop wasn't the only thing moving. The peep sight could also move--even though we tied it in place! When we put our bows in our bow cases or get our string/peep caught on something, even a tied-in peep can move.
So, we now carry one of those 6-inch rulers with us. We keep track of the measurements from "peep to nocking loop" and "peep to top of serving." This way we can determine not only if something moves, but also what moved. And anytime we start shooting high or low, we reach for our ruler.