Something very similar happened to me a few weeks ago. A multi-city that should have been bookable online was not. Called to book but my payment would never go through correctly. Eventually got it to ticket with a very helpful agent only to have BA cancel one of the four flights (LHR-PUY) the next day. I had no other booking options so had to cancel. My suspicion is that it would not let me book online due to the impending cancellation but of course cannot confirm.