Using this thread as it seems like the relevant title!

I'm trying to book a business class reward flight on BA.com, LHR-MAD-XRY (Jerez) and for the day I want, I can see there are two possible MAD-XRY flights at 15.45 and 19.40 and from the various combinations with the LHR-MAD flights offered I can see there are at least 2 reward seats available on both. From LHR-MAD there are many options on both BA and IB. I'd like to take an IB service at 10.05 and can see that when combined with the 19.40 flight to XRY there are 6 reward seats available in business class, however when combined with the 15.45 flight to XRY it shows only economy availability. Why would that be, when business class availability shows on both flights in different combinations? I'm using an AMEX companion voucher to book, if that makes a difference.