I'm not sure why that would happen. I'm assuming there is something wrong with the unit if doesn't recognize a DVD.
I would recommend that you return it to the store you purcahsed it from and see if you can get an exchange.
I just purchased a BD-P1500 and it will only play Blu-ray disks. When I put in a DVD it doesn't recognize it and says "No Disk". I did the firmware update and it still won't work.