Why is it wrong to write it without the 's' ..? i.e. vous étiez couché (not couchéS)

Grammatically without the 's' is also correct. But there might be some hint in the rest of the sentence or the test question itself which demands the 's'. Can you post the entire sentence?

Hi Chris. Thanks for taking the time to reply. Oops, I thought the whole text would be inclided, somewhere. I don't remember the text exactly but it was something like "You went to sleep early that night". There wasn't any indication that it was talking about more than one person and in fact it sounded more like it was about one person. Cheers, Peter