The .vimrc file is loaded before plugins are loaded so it is possible that a plugin is overwriting your mapping.
You can find out what the mapping was set to and where it was set by issuing the :verbose command. For example (using UltiSnips):
:verbose map <tab> s <Tab> * <Esc>:call UltiSnips#ExpandSnippet()<CR> Last set from ~/.vim/bundle/ultisnips/autoload/UltiSnips/map_keys.vim x <Tab> * :call UltiSnips#SaveLastVisualSelection()<CR>gvs Last set from ~/.vim/bundle/ultisnips/autoload/UltiSnips/map_keys.vim
If you need to override the default set by a particular plugin, you could do so by adding your mapping to .vim/after/plugin/<plugin name>.vim
(see :h after-directory).