Try changing the minimum line height field to 0.0%, my line height is also set to 0.0 but I believe that was the original default value. It probably has a value like 110% or 120% for line height now if you have not previously changed it. I believe that will do the trick. As I recall that was another tip I received from somebody else, probably taming again.
I just checked an e-pub I side-loaded earlier today and I was able to change the line spacing using the Touch slider.
Note: The line height field is on the same page - Preferences - Conversions - Common Options - Look & Feel
Last edited by fbrII; 10-07-2012 at 09:56 PM.