vim not auto popup despite asyncomplete installed

Dear all,
I installed vim version from sourceforge, then install asyncomplete
through vim-plug.
I made sure it installed properly.
However as I typed text, the auto completion popup doesn't appeared at all.
Do you have any advised about its cause?
Below is my _vimrc.
Please help me. Thank you very much.

set nocompatible
source $VIMRUNTIME/vimrc_example.vim
source $VIMRUNTIME/mswin.vim
behave mswin

set diffexpr=MyDiff()
function MyDiff()
let opt = '-a --binary '
if &diffopt =~ 'icase' | let opt = opt . '-i ' | endif
if &diffopt =~ 'iwhite' | let opt = opt . '-b ' | endif
let arg1 = v:fname_in
if arg1 =~ ' ' | let arg1 = '"' . arg1 . '"' | endif
let arg2 = v:fname_new
if arg2 =~ ' ' | let arg2 = '"' . arg2 . '"' | endif
let arg3 = v:fname_out
if arg3 =~ ' ' | let arg3 = '"' . arg3 . '"' | endif
let eq = ''
if $VIMRUNTIME =~ ' '
if &sh =~ '\<cmd'
let cmd = '"' . $VIMRUNTIME . '\diff"'
let eq = '""'
else
let cmd = substitute($VIMRUNTIME, ' ', '" ', '') . '\diff"'
endif
else
let cmd = $VIMRUNTIME . '\diff'
endif
silent execute '!' . cmd . ' ' . opt . arg1 . ' ' . arg2 . ' > ' . arg3 . eq
endfunction

let g:bookmark_auto_close = 1
if has("gui_running")
set background=light
"colorscheme solarized8_low
let s:uname = system("uname")
if s:uname == "Darwin\n"
set guifont=Inconsolata\ for\ Powerline:h19
endif
endif

" Specify a directory for plugins
" - For Neovim: ~/.local/share/nvim/plugged
" - Avoid using standard Vim directory names like 'plugin'
call plug#begin('~/.vim/plugged')

" Make sure you use single quotes


" On-demand loading
"Plug 'scrooloose/nerdtree', { 'on': 'NERDTreeToggle' }



Plug 'prabirshrestha/asyncomplete.vim'
call plug