I'm not sure exactly what you're changing?
In code use setFontTracking(). This increases spacing between all characters.
In the .fnt file (loaded automatically along with the PNG as you don't specify a file extension) you can specify
- default tracking, or
- per character spacing, or
- kerning pair spacing (between a specific pair of characters)
I've just done the credits roll for my game and I have wider/longer tracking, using setFontTracking(), so it does work.
Finally try deleting the .pdx and doing a fresh run/install. You could check the contents of the .pdx too.