pedro.thiers: you use 'set text' instead of 'append text' so you'll only ever see one hi-score. The second time 'on hi-score' triggers, for example, the text object contains the first score but Set Text changes it to the second score.
Facebook's API is notoriously buggy. If it doesn't work for a day, then starts working, it probably is Facebook fiddling with it - I had that kind of issue even when developing the plugin, which was frustrating.