Really strange. Which version of Scribus and which operating system are you using?
Generally, it is enough to select a text frame (or some text in a text frame, if you want to change line spacing only for a paragraph or two) and in Properties window select one of three options for line spacing (but only "Fixed Linespacing" allows you to change values, while other two do not). I have no problems with this on Linux, Scribus 1.4.0rc5. So, if you're using version 1.3.3.x, my first suggestion would be to try a newer version. 1.4.0rc5 works really good. And if you still have problems, then try attaching an example .sla file which shows the issue, for testing purposes.