Leaking pipe after replacing dual piped kitchen mixer tap?

Having just replaced a dual pipe kitchen mixer tap, there is a leak coming from the nut/joint which joins one of the taps pipe to the under sink copper pipe. Do I need to add or replace washers to the top/opening of the copper pipes before fastening the bolt attaching the tap to each pipe? When I originally started the job, removing the old taps, I turned the isolation screws on each pipe to horizontal but this did not appear to isolate the water in it's entirety on one pipe so I had to turn off the mains. Hopefully a red hearing but I'm wondering if this is part of the problem. I also note there are two screws in the piping near the nuts in question and can recall loosening one of them when removing the old taps. Any idea on how to resolve and stop the leak?