Pioneer A03 firmware should be 1.65 or 1.68 to deliver reliable results.
Tutorial for Nero/PrimoDVD on
www.psxcopyworld.co.uk
Now to business.
I've copied DVD-R copies to DVD-R many times and they all work perfectly in the PS2.
You didn't mention if they were DVD Movies or games. Movies need Nero 5.5.6.4 or later to be copied. haven't done it yet so can't advise further.
What are your symptoms?