As I understand it,it's a bit of a black art by a department in BA called revenue management. Essentially they release extra reward availability depending on ho sales are going on the flight. To do this they use complex modelling techniques, throw bones then consult Guru Maharanjipetit.

You can check by doing a 'dummy' Avios booking on the webiste in CW. This will tell you where there is availability. Just go through an Avios booking as normal but don't actually book the flights at the end.

The website is well known for repeatedly returning errors. You may have to persevere.