Hi,
I don't think it's a pb of decaying sensitivity of the module laser+LED/sensor, because the contrast is usually better on pressed DVDs than on burned ones.
I suspect your problem could (?) be a mechanical pb : maybe the TOC (the "catalog" zone of the DVD, near the center) of pressed DVD is closer to the center than the TOC of burned ones, and your head cannot go so for any more, if grease has gathered on its rails, at the 2 extremities of its course.
I would try a good "washing" of the rils, or at least a good exercising, with power off, to force (not violently) the head to go to the true maximum limit of its course.
my 10 cents ;-)