Try adjusting the text size up and down and see if those non-justified lines become justified. Changing the text size can eliminate the too large of a gap issue for that line, though it will likely occur somewhere else. If the non-justified lines become justified then it was just a gap issue. If not then there might be some other issue. The way many ebooks are formatted it is wonder they show up at all, so there could be some hard-coded formatting in some lines. An accidental <br /> tag can cause this.