When pullets first start to lay many of them have control over the whole process. But it is a complicated process. There are different triggers to tell them when to release a yolk to start that process so the timing is such that they lay it during daylight. Many know ahead of time to start looking for a suitable nest, others don't. Some can delay laying the egg if they need to, or know to go to the nest when the time comes. Others seem to be totally surprised that an egg is coming and drop it wherever they are, on the roosts at night or just walking around. It's not just about making the egg, it's about laying the egg. If you are finding the eggs in random spots they haven't gained control of the process. If the egg is in the same spot every day, that is where they want their nest.
I don't know why a pullet or hen changes her nest. You can speculate as to why until the cows come home and may not get it right, especially if it is only one or two. If you post photos of your nests we may be able to come up with suggestions, many of them probably conflicting. I've seen so many hens laying in so many different spots I don't think they all have the same preferences. One thing I would suggest is to check for roost mites in the nest. Mine have never had roost mites but I could see where that could cause a hen to change. I trust you have fake eggs in at lest some of the real nests? I use gold balls.
I have never tried this method but some people I trust on here say it worked for them. Put a milk crate or some kind of box with bedding where they are laying. Put a fake egg in it. After a few days where they have gotten into the habit of laying in there, gradually, a foot at a time, move that nest so they keep laying in it. Move it to where you want them to lay. If that nest is OK, leave it. If you want them in your real nests, remove that nest and hope they move to your real ones.
You can fence them out of the area they are laying in. They will have to find a new nest. The few times I tried putting stuff in their way they just laid next to what I put there, but your coop looks different to mine, it might be an easy thing to try and it might work.
I made some of my nests so I can lock a hen in there if I want to. That's come in handy a few times for different reasons. When a hen is is laying in a nest where I don't want her to I catch her on that nest and lock her in a real nest until she lays that egg. I'm kind of old and decrepit. I lock up the coop and use a fishing net to catch her. If I try to pick her off the nest she'll probably make a beak for it. No matter how excited she gets I still lock her in that nest. Usually she lays her egg within a half hour, though one hen took three hours. Most times I only have to do that once to retrain her where to lay, but for that three hour hen I had to do it twice.
