I agree with Margaid, it's most probably because they're exbatts and haven't had the chance to learn to use a nestbox, or indeed to roost in a coop and to perch overnight. Probably they've lived in confined conditions where the eggs just roll away from the floor of the cage they've been living in, so in a sense the cage is their nestbox and so they don't know what to do with all the extra bedroom space.
Do yours use the perch overnight? It's quite usual when you first get 16-week-old point of lay pullets who've always grown up in a barn to have to go down after dark and lift them on to the perch for a few nights, to trigger the perching reflex which all birds have. Another possible reason with exbatts is that their leg muscles aren't strong enough to support them on a perch, because off the overcrowded conditions they've previously been kept in. But as you say yours were well feathered and active, this is unlikely to be the case with them.
I'm down to two hens at the moment, and have arranged to get three more when I get back from our holiday next week. I was wondering, as you have plenty of room, if you had any ideas about if and when to get another couple for your flock? Its very difficult to introduce just one single hen as the others always pick on her, and similarly, it's difficult to introduce a pair to a single bird who has been left behind when her only companion died. Lovely as exbatts are, as a general rule they're not very long-lived in their new home, bearing in mind that they'll be at least 18 months old when you get them, and because they've laid so intensively they're very prone to egg laying problems such as prolapse or peritonitis which are usually fatal. So sadly it's possible that one of yours is going to pop her clogs within the next year, leaving you with a lonely singleton if you've not already got any more to keep her company.