Google Product Forums
Re: order-state-change-notification callback in sandbox not working
Google Checkout Merchant
Integration Troubleshooting :
I'm not a php dev ~
First, check if the order progresses from REVIEWING to "CHARGEABLE" (meaning Google has done its review). If its still in REVIEWING, then the order status hasn't changed (no notification)
Depending on how you integrated (XML or serial number notification), then notifications Google sends differ....