It is very common for carriers new to Pre-Check to have a changeover period.
First, BA must link up with TSA. Until then, collecting KTN's through a simple system as part of BAEC profile or PNR simply frustrates customers.
Second, Once BA is "connected", so long as the KTN is transmitted by BA, a passenger may obtain Pre-Check. This is instant.
Thurd, until a carrier can revise both its customer-facing inputs for data, e.g. BAEC profile and PNR data, this ill need to be entered either by phone or at the counter. While this means that one cannot readily bypass the counter as one might without luggage, the Carrier-TSA link is fast enough that Pre-Check will appear by the time a boarding pass is printed.
Fourth, once all of this is in place, presumably BA can modify its mobile services such that non-paper boarding documents can display (and be coded for) Pre-Check.
For most carriers, this process has taken 30-45 days.