Code snippet is not from the correct project. Each project in Convert Experiments has it's own tracking code. Make sure you add the code that belongs to the project in which you create your experiment;
Make sure the website in project settings matches the website you installed the snippet on.
Preview is showing a different page than your target defined through Site Area. That's because the preview is based on the original URL you used to create the experiment (see screenshot below); If you would like to change the preview URL, change the Experiment Page URL there.
The previous screen where you can change the URL used for preview can be reached from the experiment summary by clicking on the edit icon marked in the below screenshot:
If all the above are checked and no errors found than the problem is likely with the variation code.
If you generated the variation using the Visual Editor's functionality, than, it's possible that this does not work well on your website and you need a little help from a developer in your team
If you wrote manually the variation code, make sure the code is right, with no errors inside. Any error in the code would make it not execute therefore not render your variation.