One of the reasons could be that your plug-in’s css file is not loaded by header.php
When we build custom themes we edit header.php to include new css, scripts …etc.
In our case the header.php was rewritten from scratch, so the wp_head() php function added by wordpress in header.php template was removed.
I installed AddToAny plug-in and when I tried to get the social sharing icons by adding
I felt that something is likely preventing the icons from displaying. When I added wp_head() php function in the header.php file the AddToAny plugin css was loaded and the icons are displayed on the screen.
Here especially when you use plugins, using this wp_head() php function will insert crucial elements ( Eg: scripts, styles.. etc ) of the plug-in in your document.
Always put wp_head() php function just before the closing tag of your theme (usually in header.php)
Also some plugins add their js files in footer, so its safe to add wp_footer() before closing body tag.