This is a hard problem, so excuse me beforehand if I ask something basic:
- How did you check if the fare bucket is available? Note that availability is not the same between the True O&D and each of its segments by separate, specially between Domestic-International or Regional-Long Haul connections. Also the availability differs by ticketing country
- Is your fare still valid within the Stay and Advance Purchase restrictions?
- Did you consider the extra taxes for staying in the stopover?
If after this everything still sounds good, then you may need to HUACA. A lot of agents don't have experience for these situations