Why don't my custom snippets under ~/.vim/snippets/ directory work?

Question

I have installed the snippets plugin:

Plugin 'garbas/vim-snipmate'
Plugin 'honza/vim-snippets'

And I've put my own markdown snippets under ~/.vim/snippets/markdown.snippets. There're some triggers in it like ul which create an unordered list and ol which creates an ordered list.

But when I edit a markdown file, my own snippets not working. If I try to trigger a custom snippet from ~/.vim/snippets/markdown.snippets, the cursor flashes and erase the trigger word. But if I try to trigger a snippet from honza/vim-snippets, it still works. Please see the below gif:

What's the problem?

================================================== Below is my custom markdown.snippet

###########################################################################
#                            Markdown Snippets                            #
###########################################################################

extends pelican

#############
#  Headers  #
#############

snippet h1 "Header h1" b
# ${1:Header name}

$0
endsnippet

snippet h1i "Header h1 with id" b
# ${1:Header name} {#${2:id}}

$0
endsnippet

snippet h2 "Header h2" b
## ${1:Header name}

$0
endsnippet

snippet h2i "Header h2 with id" b
## ${1:Header name} {#${2:id}}

$0
endsnippet

snippet h3 "Header h3" b
### ${1:Header name}

$0
endsnippet

snippet h3i "Header h3 with id" b
### ${1:Header name} {#${2:id}}

$0
endsnippet

snippet h4 "Header h4" b
#### ${1:Header name}

$0
endsnippet

snippet h4i "Header h4 with id" b
#### ${1:Header name} {#${2:id}}

$0
endsnippet

snippet h5 "Header h5" b
##### ${1:Header name}

$0
endsnippet

snippet h5i "Header h5 with id" b
##### ${1:Header name} {#${2:id}}

$0
endsnippet

snippet h6 "Header h6" b
###### ${1:Header name}

$0
endsnippet

snippet h6i "Header h6 with id" b
###### ${1:Header name} {#${2:id}}

$0
endsnippet

snippet hrlnk "Header ID Link"
[${1:Text}](#${2:Header ID})$0
endsnippet

snippet hrlac "Header ID Link auto-complete"
[${1:Text}](#${1/(\w+)(\s*)/\L$1\E(?2:-)/g})$0
endsnippet

###########
#  Links  #
###########

snippet lnk "Inline Link"
[${1:${VISUAL:Text}}](${3:http://${2:www.url.com}}${4/.+/ "/}${4:opt title}${4/.+/"/})$0
endsnippet

snippet rlnk "Reference-style link"
[${1:${VISUAL:Text}}][${2:Reference}]$0
endsnippet

snippet rdef "Reference-style link ID"
[${1}]: ${3:http://${2:www.url.com}}
$0
endsnippet

snippet flnk "Fast link"
<http://${1:url}>$0
endsnippet


##############
#  Emphasis  #
##############

snippet * "Italic"
*${1:${VISUAL:Text}}*$0
endsnippet

snippet _ "Strong"
__${1:${VISUAL:Text}}__$0
endsnippet

snippet *_ "Italic and Strong"
***${1:${VISUAL:Text}}***$0
endsnippet


############
#  Quotes  #
############

snippet > "Quote" !b
 > ${1:${VISUAL:Text}:${VISUAL:Text}}

$0
endsnippet


###########
#  Lists  #
###########

snippet ol "Ordered List"
1. ${1}
2. ${2}
3. ${3}
4. ${4}
$0
endsnippet

snippet ul "Unordered List"
* ${1}
* ${2}
* ${3}
* ${4}
$0
endsnippet


######################
#  Definition Lists  #
######################

snippet dl "Definition List" !b
${1:Word 1}
: ${2:Definition}
${3:Word 2}
: ${4:Definition}
${5:Word 3}
: ${6:Definition}
${7:Word 4}
: ${8:Definition}

$0
endsnippet


############
#  Images  #
############

snippet img! "Image"
![${1:pic alt}](${2:path}${3/.+/ "/}${3:opt title}${3/.+/"/})$0
endsnippet

snippet ilnk "Reference-style Image"
![${1:${VISUAL:Text}}][${2:Reference}] $0
endsnippet

snippet idef "Reference-style Image ID"
  [${1}]: ${2:path}${3/.+/ "/}${3:opt title}${3/.+/"/}
  $0
endsnippet


############
#  Tables  #
############

snippet table "Table"
${1:First Header}   | ${6:First Header}
--------------------|--------------------
${2:Content Cell}   | ${7:Content Cell}
${3:Content Cell}   | ${8:Content Cell}
${4:Content Cell}   | ${9:Content Cell}
${5:Content Cell}   | ${10:Content Cell}
$0
endsnippet


##########
#  Code  #
##########

snippet ` "Inline code"
\`${1:code}\` $0
endsnippet

snippet ~ "Code" b
~~~
${1:${VISUAL:Code}}
~~~
$0
endsnippet


#####################
#  Horizontal Rule  #
#####################

snippet hr "Horizontal Rule" b
----------

$0
endsnippet


###############
#  Footnotes  #
###############

snippet ft "Footnote Reference"
[^${1:Footnote label}]
endsnippet

snippet ftd "Footnote Definition"
  [^${1:footnote label}]: ${2:Footnote}
  $0
endsnippet


##################
# Abbreviations  #
##################


snippet abb "Abbreviation" b
*[${1:abbreviation}]: ${2:definition}
endsnippet