Well multiple things could be wrong here. First, are you certain that you have a male and a female pair? It is also possible that you have an infertile bird. If your finches happen to be related they could be laying eggs that due to the inbreeding will never successfully hatch. Try pairing them off with different partners and see if this makes any difference. As long as your flight is larger enough to accommodate another pair of finches there is no reason why you can not have them all reside together. Just make sure you only add more finches to the flight and not another type of bird!
