HELP! My chicken has started laying from its roost

I have restarted a small flock after several years off and have five chickens of varying ages before laying maturity. Our first chicken started laying last week (success!) and was using her nesting box as hoped. She laid about seven eggs over seven days in the box but then the last couple of days I have found her daily egg smashed beneath the roost.

I have read it is normal for some chickens just starting to lay to do so from the roost as they get used to the whole egg coming out of them thing but am wondering if I have a problem with this hen given her first several eggs were in the nesting box without any drama.

Is she getting into a bad habit or did I just luck out with the first week and she has stumbled over the last couple of days?

My nest boxes have a plastic egg each to encourage laying there and there is no problem with mice, rats or other pests that might have spooked her.

If her first few eggs were under the roost I wouldn't be worried because I know she's learning but because she started so well I just don't know what has happened to change things.

Any and all advice much appreciated!