Some people and accounts have been having trouble booking avios tickets for other people while other people and accounts have not. Some of the issues seem to date back to the rebooting of the BAEC system last fall. Here is one of the threads:
https://www.flyertalk.com/forum/brit...-person-2.html
Also mentioned occasionally in other threads including the bugs thread:
https://www.flyertalk.com/forum/brit...r-free-36.html