We are flying to Gran Canaria and tried to book seats on the Iberia segment. BA takes you through to Iberia on manage your booking but I couldn't choose any seats. I then went through Iberia directly and it didn't recognise either the BA or Iberia booking reference. Apparently this does happen to others quite frequently. If you have found the same, trying accessing the booking on Qatar. It works! Bit ridiculous but never mind!