Why some pages show schema and some don't in Google?

[rbai]

I notice Google displays the schema(reviews, price, availability etc.) in results only for some of our item pages in same category using same template. Any ideas why this is happening. They are created around same time - more than a year ago. Schema was also added a year ago.

[DirkC]

Even if you implemented all the tags correctly there is no guarantee that Google will show the them in the SERP's - the choice whether or not to show them is entirely up to them.

I guess you already checked if all the tags are properly implemented (if not - check here or here)
You could also check this page for the usage guidelines (bottom of the page).

Again - even if you are doing everything correct & you meet the guidelines - it's entirely up to Google if the snippets will be shown or not. You can't control it.

Dirk