Spacing in script writing mode

Hi,
I must admit I don’t really understand how spacing work in script writing mode… For example, I want to do that :

blabblalalalalalalalaalalalalalalalaalalalalalalalala
blabblalalalalalalalaalalalalalalalaalalalalalalalala
blabblalalalalalalalaalalalalalalalaalalalalalalalala

   [b]  blblblblblblblblblblblblblblblbl
     blblblblblblblblblblblblblblblbl
     blblblblblblblblblblblblblblblbl[/b]

blabblalalalalalalalaalalalalalalalaalalalalalalalala
blabblalalalalalalalaalalalalalalalaalalalalalalalala
blabblalalalalalalalaalalalalalalalaalalalalalalalala

And I have that :

blabblalalalalalalalaalalalalalalalaalalalalalalalala
blabblalalalalalalalaalalalalalalalaalalalalalalalala
blabblalalalalalalalaalalalalalalalaalalalalalalalala
blblblblblblblblblblblblblblblbl
blblblblblblblblblblblblblblblbl
blblblblblblblblblblblblblblblbl

blabblalalalalalalalaalalalalalalalaalalalalalalalala
blabblalalalalalalalaalalalalalalalaalalalalalalalala
blabblalalalalalalalaalalalalalalalaalalalalalalalala

And my spacing are (for the bit in bold) :
Before : 12,00 pt
After : 12,00 pt
line spacing : Exactly
At : 1,00

Sometimes, when I enter new spacing (Before or After), the new parameter is not saved…
Any help will be much appreciated.
Thanks!