Sticky Button not working WP. But does it JS Fiddle

It might be a syntax error inside your custom CSS section. Try pasting the whole CSS code (not just the snippet above) to a code editor which can give you hints as to what the error might be – if there is one.
Or it could also be an HTML syntax error – you could check your page’s source code for missing closing tags or other similar errors that could lead to the page not displaying properly.