Planning a flight is odd when the tickets are more often reserved than sold...  i guess it' like reserving a seat in a Restaurant... easier to book and cancel than to miss out.

I was told that travel agents book seats for various reasons such as 
Confirm a price to the client
Get exclusivity on a flight or travel date then package the tour (like holidays) 

but none of it is valid until the deadline to pay which is a few days before the flight and then the seats or bookings are released and seats on the flight become available .. so when seeking a ticket and informed "no seats" it can mean "reserved" but not sold

If you travel in small groups ie., 2 or 3 people you can take a chance of getting on the flight even though "no seats" you can ask for "standby" and re check a day or 2 before the flight ...  this can be successful because there is a good chance many tickets are reserved (no sold)  obviously it gets a little tricky during peak periods. 

If you travel in large groups it's too risky (that is not that many reserved tickets will be released) so you should book well in advance

The smart move is to join the loyalty membership group with the airline because this gives you priority over other passengers - in the link below for the TG flight schedule there is a link to join ROP (Royal Orchid Plus) which is TG's member club.... 

Maybe a travel agent out there can spread some light on this because there a lot of myths about this process

What I do know is Travelling between Thailand and West Australia there is only one (1) WAY TO TRAVEL AND THAT'S ON THAI AIRWAYS....