Well, I checked the margins, and with each button image being 119 pixels wide, the margins of 50 for the left and 59 for the right are just fine so there is no issue there.
I would recommend applying a different skin, reboot your computer, and then go to C:\Users\Public\Documents\Stardock\WindowBlinds and right click on the PlasFan folder and delete it. Then reboot again and re-install the Plastic Fantastic .wba file and apply.
There could possibly be something corrupted in the skin and that should fix it.
Hope it helps, let me know. 