If you're on daily or weekly, you need to set the time to 09:30am for US markets (use the debug mode to find out the correct time for any other markets). If you're working on intraday than set it to whatever time you want.
This is a fault of tradingview - it only lets you calculate so much data. I have found if you use it at non-peak times it works... but most of the time if you're calculating a lot of data it wont work. Use TC2000 if AVWAP is central to your trading.