with 70,000+ NEM shown in my vested balance I’ve only harvested 3 blocks a month ago and every following block has been 0 fees earned. Is there an error I’m possibly making or is this just bad luck?
Everything is determined by luck. 
Kind of poor luck but entirely possible.
Same issue here i guess. I am delegated harvesting on a node in EU. I setup it up a week ago, it harvested 1 block the first day with 0 fee return and no blocks since. Surely this works on more than just luck? Whats the logic in the harvesting process i.e. whether a block is harvested or not and whether i get a fee or not? Also do i need to keep my windows machine active? I assume once i select a node and start the harvesting its running on that nod even if my nem client is closed?
once a node is activated and has 10,000+ vested balance you do NOT have to continue running the wallet on your computer to continue harvesting.