Highlighting not working--or I don't know how to do it

I have had no problems highlighting Logos on my PC. But I curled up in bed to read a fantastic new book I purchased by Leland Ryken on my iPad, The Word of God in English. I have the highlighting function on in settings. I have been trying to highlight and succeeded to a small degree. But I cannot consistency get my highlights going. What is meant by swiping in this case? Any help would be most welcome.