Looking at the browser console can reveal jQuery not defined errors in red.
To view your browser console press Command+Option+J (Mac) or Control+Shift+J (Windows, Linux, Chrome OS) to jump straight into the Console panel of Chrome DevTools.
This may be due to how the Autoptimize plugin was configured. You should follow the exact settings outlined in the screenshot in our speed tips guide.
Be sure to look at the Exclude scripts from Autoptimize section and ensure that jQuery is excluded.