Hawaii is a tough destination for award tix. Last year I did a trip to HI using US miles and flying entirely on UA metal. I had to book at 330 days ahead and I still needed to flex my travel dates by a few days to get the awards. I also had to fly mainland to Oahu, even though my destination was Kona. There were no outbound award flights to Kona open for awards. I managed to get Kona to mainland on the return. The open jaw is not such a bad deal since you get to see more than one island and the inter-island flights are only about $80 per person one-way IIRC.

I tried various methods (online, US phone agents, UA phone agents just to see if they showed inventory for their own members), and in the end the US phone agents came through for me.

Also some folks on FT and the res agents I spoke with suggested trying again a few months later since seats do sometimes open up as dates get closer. You just have to be flexible about dates.