I just figure out the problems.
1. signature not showing at post categories
This is because the theme Nexus uses "the_excerpt" function, instead of the recommended "the_content" function.
I add some quik-fix code in the signature plugin. Now the signature shows at post categories.
I will release a new version of the plugin later. Please update then.
2. signature not showing at some posts
This is because these posts are in more than one categories, such as "5-En vedette", which is not checked to the option "Which categories should the signature be placed?"
The strategy is: those posts in the categories which is not checked would not have the signature, even if they are also in another category which is checked.
So, that's all. Thanks for your feedback. Please let me know if you still have any questions.