In most cases when the button isn’t showing it’s because something is layered over the top of it with a higher z-index.
In the plugin settings there is a field where you can define z-index for the buttons. An element with a higher z-index is always in front of an element with a lower z-index. If you have tried a high z-index in that setting but your buttons still aren’t showing, it is likely that your theme has a very high z-index specified somewhere. This is not great practise, but is sometimes done in complex themes with lots of layered design elements, and also with page builders. As a last resort you can use CSS to force override the z-index for your buttons: #ssb-container { z-index: 9999 !important; }
If you share a specific example I can use the element inspector to try to work out the culprit on any given site. But without a link to the affected site, it’s impossible to give more help than the general pointer above per the FAQs.