This is a very common issue with Porto Theme on Magento 2.x.
You might get the following errors in your console while you are on the product detail page:
Uncaught TypeError: $(...).swMegamenu is not a function black-leather-jacket.html:429
Uncaught TypeError: Cannot read property ‘RFC_2822’ of undefined theme.js:27
Uncaught TypeError: $(...).stellar is not a function dataPost.js:13
Uncaught TypeError: $.widget is not a function
This is an issue with the theme itself, and by their recommendation, you should go to Admin > Stores > Configuration > Advanced > Developer, and turn “Merge JS” to Enabled.
Flush your cache and your JS/CSS Files, and that should do the trick.