Inconstant table borders when viewing in a pdf

Hi - I am having trouble with table borders when viewing a pdf. I am using css to format the table. I have no formatting in the html its self. Yet I have one line that looks thicker then all the others. It isn’t consistent. One pdf will have one line thicker and the nest might have every other line thicker. This happens all the time. Is there any advice or a special trick to this?

Thank you
Amy

Can you show a screen shot of the PDF issue?