As others saying you need to sort this out with the TA. Also BA is a UK airline so the UK261 law is applying. That means if no airline has availability then you can claim cost of extra nights in hotels plus food, etc from the operating airline. I have done this before and BA paid without much of a problem. You haven't mentioned the size of the group, also as you booked with the TA what ticket type were used, etc. Generally 9 or more is group booking and you can't really see proper seat availability. If it is just 4-5 then online tools should work.