Related blog posts not working

I've being checking this post and It don't output anything at:

perch_blog_post_categories(perch_get('s'));

and it output Array() at

print_r(perch_blog_post_categories(perch_get('s'), array('skip-template'=>true)));

Debug Message

[1] SELECT * FROM perch2_blog_posts WHERE postStatus='Published' AND postDateTime<='2016-02-28 10:37:00' AND postSlug='probandolo'
[22] SELECT DISTINCT settingID, settingValue FROM perch2_settings WHERE userID=0
[1] SELECT * FROM perch2_blog_sections ORDER BY sectionTitle ASC
[1] SELECT * FROM perch2_blogs ORDER BY blogTitle ASC
Using template: /templates/blog/post.html
Using sub-template: /templates/blog/post-form.html
[1] SELECT SQL_CALC_FOUND_ROWS DISTINCT tbl.* FROM ( SELECT idx.itemID, main.*, idx2.indexValue as sortval FROM perch2_blog_index idx JOIN perch2_blog_posts main ON idx.itemID=main.postID AND idx.itemKey='postID' JOIN perch2_blog_index idx2 ON idx.itemID=idx2.itemID AND idx.itemKey='postID' AND idx2.indexKey='_id' WHERE 1=1 AND ((idx.indexKey='postSlug' AND idx.indexValue='probandolo')) AND idx.itemID=idx2.itemID AND idx.itemKey=idx2.itemKey GROUP BY idx.itemID, idx2.indexValue ) as tbl WHERE (postStatus='Published' AND postDateTime<='2016-02-28 10:37:00' ) GROUP BY itemID, sortval ORDER BY sortval ASC LIMIT 0, 10
[1] SELECT FOUND_ROWS() AS `count`
[1] Using template: /templates/blog/slugblog.html
[3] SELECT regionKey, regionHTML FROM perch2_content_regions WHERE regionPage='/viaje.php' OR regionPage='*' ORDER BY regionPage DESC
[1] SELECT SQL_CALC_FOUND_ROWS DISTINCT tbl.* FROM ( SELECT idx.itemID, main.*, idx2.indexValue as sortval FROM perch2_blog_index idx JOIN perch2_blog_posts main ON idx.itemID=main.postID AND idx.itemKey='postID' JOIN perch2_blog_index idx2 ON idx.itemID=idx2.itemID AND idx.itemKey='postID' AND idx2.indexKey='_id' WHERE 1=1 AND ((idx.indexKey='postSlug' AND idx.indexValue='probandolo')) AND idx.itemID=idx2.itemID AND idx.itemKey=idx2.itemKey GROUP BY idx.itemID, idx2.indexValue ) as tbl WHERE (postStatus='Published' AND postDateTime<='2016-02-28 10:37:00' ) GROUP BY itemID, sortval ORDER BY sortval ASC LIMIT 0, 10
[1] SELECT FOUND_ROWS() AS `count`
[1] Using template: /templates/blog/post.html
[2] Using sub-template: /templates/blog/post-form.html
Fetching from cache: perch_blog_post_categories0de041f4abb7c49c3ee005bf9bbac8e3
Caching: perch_blog_post_categories0de041f4abb7c49c3ee005bf9bbac8e3
Fetching from cache: perch_blog_post_categories9d886013b1e484b3e7fbbda010cb4af4
Cache file not found: perch_blog_post_categories9d886013b1e484b3e7fbbda010cb4af4
[] Using template: /templates/blog/category.html

Summary information

Perch: 2.8.25, PHP: 5.4.4, MySQL: 5.5.25, with PDO
Server OS: Darwin, apache2handler
Installed apps: content (2.8.25), assets (2.8.25), categories (2.8.25), perch_blog (5.0), perch_forms (1.8.3)
App runtimes: <?php $apps_list = array( 'content', 'categories', 'perch_blog', 'perch_forms', );
PERCH_LOGINPATH: /perch
PERCH_PATH: /Applications/MAMP/htdocs/eurovacaciones.es/perch
PERCH_CORE: /Applications/MAMP/htdocs/eurovacaciones.es/perch/core
PERCH_RESFILEPATH: /Applications/MAMP/htdocs/eurovacaciones.es/perch/resources
Image manipulation: GD
PHP limits: Max upload 32M, Max POST 32M, Memory: 64M, Total max file upload: 32M
F1: dc1fef2ad0fcd9f943c02ebb43d85dbc
Resource folder writeable: Yes
HTTP_HOST: eurovacaciones:8888
DOCUMENT_ROOT: /Applications/MAMP/htdocs/eurovacaciones.es
REQUEST_URI: /perch/core/settings/diagnostics/
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php