|Our lawn is little by little going to|
|I agree with the advice you've been given. Sod often has a biodegradable netting which may or may not biodegrade in a few years. The netting may be holding thatch or dead grass roots which could potentially keep the seeds from making good contact with the soil. The seeds may germinate right on time but without establishing a root system in the soil, it will perish. I'd rake the dead area, work in a little compost, then overseed and cover the seeds with peatmoss or compost to help keep them moist. The seeds should sprout and send their roots deep down into the soil. Be sure to use fresh seed so you get 90-100 percent germination.
Best wishes with your lawn!