Thanks for that Jax.
Yes, it's frustrating when the 'not available' thing comes up.
About two weeks ago I was trying to bookflights Went to Thai Airways Head Office in Bangkok.... no seats, tried a travel agent.... no seats, tried Singapore airlines website and kept getting the no seats match your details (or something like that... although I'm sure that this couldn't have been the case with ALL the dates I put in - and I put in many!) No luck, gave up.
Got another travel agent to do the job and BINGO - got flights on Thai.
He explained that one of the reasons for 'no availability' with the non-discount airlines is that travel agents book blocks of seats and on-sell them (to tour groups/package tourists etc.) as a "group fare".
I guess this means that if the agent doesn't sell them all to groups, they'll be made available to individuals. All that we have to know is which agents can sell us the seats!!