Facebook Fan Box issue

The facebook’s fan box (directly embedded, without plugin nor widget) is presenting a strange behavior in one of my webpages: https://paiz.cl

It is embedded in the sidebar template and displays nicely in homepage and all the pages:

• Authors: https://paiz.cl/author/daniel-gimenez/
• Categories: https://paiz.cl/publicado-en/columnas/
• Tags: https://paiz.cl/temas/consenso-de-washington/
• Archives: https://paiz.cl/2011/09/

However, it doesn’t display at single posts: https://paiz.cl/el-sistema-chileno-de-partidos-for-dummies/

I already modified the code for sidebar and single templates in all imaginable ways, but cannot solve the issue.

Any help will be welcomed…