A couple of days ago I booked an Avios redemption for a family member, for FRA-LCY tomorrow. The booking has ticketed normally but they have no seat selected as they're a "mere" BA Blue.

​​​​​API has been completed successfully but on trying to check in, BA.com is saying OLCI is unavailable.

Does anyone know why this could be? The flight looks to be wide open (Y9 and plenty of seats that are free on EF) so I hardly think it's overbooked. And I've just checked myself in for a FRA-LHR flight tomorrow (and have done so online previously) so it's evidently not a general problem with OLCI at FRA.
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Thanks!