It is possible that since you didn't select seats at the time of booking, there are no more seats that can be pre-assigned before the day of the flight. If you are flying relatively soon, you can continue trying to get seat assignments before arriving at the airport. If that doesn't work, I recommend getting to the airport around 3 hours before your flight so you can assure yourself of seats on the flight. It sounds like the flight is probably oversold, and some people won't get seats without others giving up their seats.