This plugin works perfectly for me when I use it on a regular page. I would like to use it on a tabbed page, though.
The theme I use has a shortcode that creates a tabbed page. When I nest the '[simplemap]' shortcode within the shortcode for the tabbed page, the map for SIMPLEMAP doesn't display at full width. (It only shows a small portion of the map, in the upper-left corner. Look under 'Locations' on this page... http://thecomputerstop.net/store-locator/)
I've tried using a plugin that is supposed to allow you to nest shortcodes. I get the same result with or without the nesting plugin.