They need to enter all flights starting with the first one from scratch. This will establish AGP as PoC. All availability after that will follow, including any availability restricted to married sectors which would otherwise not be available. I suspect BA has not adequately trained all its agents on this.

You might want to call to make a new booking, then once you have all flights you need, tell the agent you now want to change your existing ticket to the new itinerary. Make it their problem and not yours.