This is because of the difference between a reservation and a ticket, which can be baffling.

When you phone to change the booking, what the phone agent changes immediately is the reservation. This is what shows in MMB, and in most cases allows you to use the MMB functions.

However, the ticket for those flights may or may not have been issued (for a new booking) or re-issued at that point in time. In the case of a changed booking, the re-issue process can take some time; and even for new bookings, there are some travel agents who delay issuing the ticket until very close to the time of departure (because, AIUI, ticket issue is the point at which the TA becomes obliged to pass the money on to BA).

In fact, sometimes the ticket hasn't been issued or re-issued by the time that OLCI opens, which is one of the reasons that some people find that they can't OLCI at that point.