Abode Reader highlight function works intermittently

I am trying to use the highlight function on Adobe Acrobat Reader on my Mac Book. I am working with a downloaded textbook, not a scanned image of a text. When I select the highlight button, it lights up yellow and the curser turns into a text bar with a dashed box around it, however it doesn't always highlight the text when I scroll over. Oddly enough, any text that is located in a image is able to highlighted every time. The odd this is, if I randomly click around on the page or scroll down to a new page, the highlight function will work. Then when I try again, without changing anything, it no longer highlights. Another odd point is that, if I click on the highlight button on the toolbar, it won't turn off, as in it stays yellow. Any suggestions.