All items that are available on the Envato Marketplaces go through a review process by quality workers (they’re human real-workers and as humans we all make mistakes from time to time, but hey! that’s life) and then they determine if it is suitable to be available for the marketplace to ensure the item works as it states and also ensuring all the functionality is up to a professional standard.
The good thing for buyers is that all themes available on the Envato Marketplace have a demo online which you can test and trial out some functionality alongside this some authors of the community even make their documentation publicly available - which is great as you can see the nitty gritty details!
The first option I would recommend you do is check if the author supports their item and if they do if you contact them through the “Support” tab on the item page they will be able to assist you in the best possible manner and give you the most accurate results.
However, if you believe the item does not function as it states on the item page with the incorrect functionality if you contact the Help Team they will be able to issue you with a possible refund and help solve any problems.
I hope this helps and good luck!