If you are logged in as an admin to your store, Shopify puts a black admin bar over the top of the page which can hide the bar. Regular customer would not see the admin bar, and when you log out of your shopify admin it will disappear.
Another potential cause is that some themes will use an AJAX add to cart function where the user stays on the product page when the add to cart button is clicked and the page doesn't refresh. In order for the motivator bar to appear the app requires a page refresh, so the motivator bar wont appear until the user goes to another page. Unfortunately it's not possible to have it fire on "add to cart".