When they first start to lay, they don't always have control over the process and may drop one anywhere, anytime, even when on the roosts. This is usually not a big problem. Once they gain control over this process, they look for nests to lay them in.
However, sometimes they decide some spot other than the nesting box is a good place to lay. If they are in the habit of laying somewhere else and expecially if they have formed a nest there, you may have a problem. Sounds like this may be your case.
I find it helpful to teach them where to lay to put a fake egg in the nesting box. A golf ball, wooden or rock egg, or plastic Easter egg can work. Chickens like to lay where others are laying. If yours are in the habit of laying somewhere else, this might or might not retrain them. It does not work for me when I have one laying on the floor, but some people on this forum have posted that it worked for them.
I built my nests so I could lock a hen in there if I wanted to. Or lock them out for that matter. I over-design and over-build things. That is just me. When I have one regularly laying on the floor, I go by a lot and when I catch them in that nest on the floor, I lock them in the nesting box until they lay the egg. That usually takes a half hour but some take longer. Usually doing this once teaches them where to lay, but I had one stubborn hen that took over three hours to lay her egg, then I had to do it again the next day. Eventually, she did learn.
Good luck with this. It can be a little frustrating dealing with it.
tia
