Cannot highlight using Acrobat Reader Pro DC version on iPad

Hello.  I have not been able to use highlight feature at all using the iPad Acrobat Reader at all for some time.  It's irritating me a lot. I pay for a DC subscription and you'd think the applications would work correctly.  I tap the pencil Botton, I tap comment, I tap highlighter, and it does nothing when I use my pencil or my finger except magnify the letter on the pdf.  What gives, experts of all things adobe?????????