A bit late but I had this issue as well. As you're using fzf.vim you can configure the Rg command as per their instructions by adding the following to your init.vim file and adding in the --hidden option.
command! -bang -nargs=* Rg
\ call fzf#vim#grep(
\ 'rg --hidden --column --line-number --no-heading --color=always --smart-case -- '.shellescape(<q-...