I have 4 warren Hens all at pol some young some older but only one is laying can anyone suggest why?

They're in a covered pen 12' x 10' have a roomy coop, they're fed on layer's pellets and fresh water daily plus lettuce, bread and mixed corn, so what am I doing wrong? have I been supplied duff birds?, unfortunately they came from a reputable dealer. 2 of the birds look older than the other. Any suggestions?