If you are logged in as an admin, Shopify puts a black admin bar over the top of the page which can hide the Sales Motivator bar. A regular customer would not see the admin bar; logging out will correct this problem.
Another cause is that some themes use an Ajax add to cart function where the user stays on the product page when the add to cart button is selected, and the page doesn't refresh. For the motivator bar to appear the app requires a page refresh, so the motivator bar won't appear until the user goes to another page. Unfortunately, it's not possible to have it fire on "add to cart", you would need to disable the ajax cart for Sales Motivator to work.
One of two things can be done to fix this:
- Disable the Ajax Add to Cart Functionality in the Theme; or