It can take months to pursue a claim that hasn't tracked at all but you should be able to ask for an update quicker if the cashback has been rejected by BT. But at the end of the day cashback is never guaranteed & sometimes valid claims are rejected because "Computer says no". TCB also have a discretionary fund where they can make you an offer of some of the cashback that was due if you make enough of a fuss.
I find that over the long term, sometimes valid claims are rejected but also some that should be rejected are paid. Cancelled/returned orders for example. So it broadly evens itself out over time.
