CJEMNMAC is right on the money.
In addition to the comments, it's important to note that award seats are NOT to reward you for being a loyal, or valuable customer.
They are there purely as a mechanism to drive revenue to the loyalty program and airline.
The loyalty program (Velocity, in this case) wants you to take out more credit cards.
The airline (Virgin Australia, in this case) is allowing the purchase of seats using Velocity Points in place of cash, to fill the planes.
As such, the airline will release award seats when the flight is under-sold, or the chances of selling a seat for cash are lower than usual.
There is no requirement, nor incentive for award seats to be released at set time-frames.