When I made our seat requests last week I spent a while trying to work out why the web site kept rejecting my request. I was trying to be too clever and split the party, to occupy a number of rows, so that I could come back with the other booking (for the SIL and BIL) and fill in the seats.
Eventually I twigged that the web site was not letting me sit DS (13) by himself. He has to be sat next to an adult. Once I did this the rest was easy. I ended up making the seat requests for both parties in two web browsers at the same time.
This is the tack I'll use when it comes time to OLCI.

Chris