It is hard to get schema properties right, because Google is changing how it uses it for its search results all the time. A year back I optimized some product

It is hard to get schema properties right, because Google is changing how it uses it for its search results all the time. A year back I optimized some product pages so that they displayed the price and availability in the search results. Very nice. Then, two weeks later, Google changed something and the pages stopped showing their special properties and were back to normal in the search results. No idea why...