PlantUML doesn't show

I install this plugin using `packer` like this:
```lua
"iamcco/markdown-preview.nvim",
run = function() vim.fn["mkdp#util#install"]() end,
setup = function()
vim.g.mkdp_filetypes = { "markdown" }
vim.g.mkdp_open_to_the_world = 1
vim.g.mkdp_echo_preview_url = 1
vim.g.mkdp_open_ip = '10.160.209.78'
vim.g.mkdp_port = '65532'
vim.g.mkdp_refresh_slow = 1
vim.g.mkdp_theme = "light"
vim.cmd(
[[
function! LemonadePreview (url)
execute "silent ! lemonade open " . a:url
endfunction
]]
)
-- vim.g.mkdp_auto_start = 1
vim.g.mkdp_browserfunc = 'LemonadePreview'
vim.g.mkdp_auto_close = 0
end,
```
and use the example in `README` to test :
```txt
``` plantuml
Bob -> Alice : hello
```
```
when I run `:MarkdownPreview`, the webpage show up, but the diagram doesn't show up.

by the way, mermaid works fine.

What's wrong?