.kas files are created by users. They are created for particular version of lyrics so most common reason for kas file "not working" is that you have different version of lyrics. You can use "try next" button to load the version of lyrics karaoke file was created for.
Sometimes only some lines don't get highlighted, you can use ELLO to edit kas file timestamps. Get more info here
Other problem can be that highlighting is just lagging behing what's being sung. You can use little arrows at the botton of the window to shift the time
If you tried all the methods and none of them worked and you are positive that .kas file is wrong you can let us know by clicking on EvillLyrics statusbar and vote negatively for that karaoke. You are also encouraged to vote positively if karaoke works for you. It helps us to choose best karaoke files.