You might need to embed the bold font, your image shows 'Regular' font.
Ok, I figured out the magic here. Likely why it started working for you is that you actually set the bold font for your field.
But here's the magic. In the library, right-click on the bold font and choose properties. Then go to the actionscript settings and enable :
export for actionscript
Next, in your code you need to add this line :
BoldTextFormat.font = "Times New Roman Bold";
That should work.
Sadly, the .bold of the TextFormat doesn't seem to automatically find the bold version.