There is a sale on at the moment as well! I have noticed many times that while the sales are on the avios seats available reduce quite a bit!

Maybe wait until the sale finishes and check back a few days after that! I have also noticed that availability switches much more these days than it used to. One day there is loads or seats, the next hardly any to some places but then the next week there are lots of seats available again!

Keep checking it could open up!