Fiji Airways Seat Selection when you are flying as Qantas Codeshare Pax

[here2go]

My family and I are flying to Fiji next month on a Fiji Airways flight ex CBR, but QF Flight number booked through a package holiday provider. I've tried selecting seats through the Fiji Airways website, and there are bucketloads there, but I keep getting error messages once I have selected the seats. Is this normal? I've flown Fiji on a non-codeshare flight in the past with no issues, so am wondering if the fact this is a QF booking is causing the issues.