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Singapore Airlines seems to reserve a large number of seats further forward in the aircraft, which are not available for selection at booking time. When you check in, you will be able to select a seat. On the flights where you have already been able to select a seat, you will be able to switch to a more desirable seat if you wish. Check in as early as you can (48 hours before the flight), to ensure a good selection of seats. If you don't check in online, you will still be allocated a seat, but it probably won't be a good one.
This answer is from my own experience on Singapore Airlines.