'Invisible' Print - Not a mark on the page.

Most likely the purge assembly. That's the unit where the printhead parks and it also primes the printhead for printing. If you run a clean cycle with an ink cart. removed and see if the ink spot is cleaned from the ffilter in the printhead, the purge is working. If you can see ink after a cleaning the purge assembly needs to be replaced. About $85.00 in a shop.