unable to use clone after adding text

[picbyer*]

I just can't figure out why, after adding text to a doc, I can't go back and use the clone tool. I am trying to clean up a scanned document that has been photocopied too many times. It is a plot map of a cemetery and I am removing the dark background/numbers but leaving the lines of the individual plots. Then I go back and replace the hand written plot numbers with Text numbers. But when I go back to try to use clone to work on another row, I can't get it to use the selected clean portion to "erase" the dark pixels. I think it may be something to do with layers but I don't know how to use them. I've not found the correct tutorial for this yet. Hope someone can help without getting too technical.
Thank you