Sitting but no egg

One of our ex-batts, with us five weeks now, often goes up into the nest box and sits there for a while, but I've not had an egg from her for quite a few days now. I've checked her vent and there's no sign of an egg, and she's not off-colour in any way.

I take it she may simply have stopped laying? We had a few odd eggs that I thought might be hers a couple of weeks ago. Since then, I can't be sure, but I don't think she's laid ... we have four ex-batts and I'm getting a regular three eggs a day, but she's the only one who sits on the one side of the nest box and I haven't found an egg over there for a while.

Any thoughts?