_x not working localizing custom post type and taxonomy

I’m trying to localize a plugin that creates a custom post type and a custom taxonomy for that post type.

it’s everything working nice, except from strings contained into _x expressions.

if I use __ and _e expression it’s ok,
if I use _x to define also a $context arg translation is not showing.

naturally when creating the pot language archive file I scanned also for _x and not only for __ and _e

summary:

this is working : _e( $text, $domain ) , __( $text, $domain )
this is not: _x( $text, $context, $domain )