Bill,
I think what you mean is that natural law ethical theory as it has been used in Catholic moral theology since the high middle ages, is problematic. That method of natural law moral reflection is fundamentally "deductive"--it purports to start from unassailable general principles, and from these deduce moral prescription. 

I believe that our widely accepted codes of human rights are also derived by natural law reflection, but the process is "inductive"--one starts with concrete factual data, and from this attempts to rise up to moral prescription.

There is a third version of natural law used frequently by many bishops...you assert a moral conclusion is part of natural law, and expect that to end discussion.

(I began to illustrate my point with examples, but ran into the character limits of this new system...bummer.  I think Civil Comments website suggested replying to oneself as the device to expand the length of a comment, but I'm not ready to try that just yet.)