QR are very good at doing this. It is likely you were just unlucky that one time. I have done multiple QR to BA separate ticket interlining baggage without any issues.

BA will accept and process any baggage that is tagged onto them. BA just won't interline baggage across separate tickets if they are the starting carrier in the journey. This thread has extensive posts about QR interlining separate tickets:
QR Interlining (Baggage)