Hen wont use nesting box

Have two hens Gertrude and Hilda that are new to me.

Thought they took along time to come into egg production and then found egg remains outside house in the run.

Put this down to the fact that once a piece of string had snapped dropping the pop hole door and effectively locking them out of the house.

Sorted this and seemed to be getting an egg a day in the same nest box.

Eggs varied in size one day big next small so presume girls are laying alternatively as Hilda is bigger slightly than Gertrude.

They also seem to lay middle of day as if I visit early sometimes nothing there.  Yesterday there where no eggs and as this has happened once before thought nothing of it. Today when I go down usual nest box has smallish egg and nest box by it has a smallish egg. Third and final nest box nearest door has nothing. However in the run I found a dirty smallish egg which looked like it had been layed a few days as it looked sun bleached and I had not seen it till now.

If a nest box has an egg in it do the hens go to the next free one. What is wrong with the third nest box I know its near the door but surely must be safer than laying in open.

Any tips to get them laying inside house.

Thanks Alan

Put a couple of golf balls in the nest box and they will want to add to the clutch.

Put a couple of golf balls in the nest box and they will want to add to the clutch.

Thanks Auntie.  Will try that. However yesterday had my worse game off golf ever and need to buy some more golf balls first.