I had a similar experience with ITA earlier this year. Normally I do exactly as rferguson does, and usually it works.
Earlier in the year I saw an excellent CX Business class fare on ITA from LIS>HKG. I tried many websites, including CX, then remembered what we used to do 10 years ago, and I telephoned a travel agent. I gave them the exact ITA routing, dates and class and, Hey presto, the agent booked it.
I had to pay a travel agent fee and credit card surcharge, but it was worth it.