Full Idea
The two criticisms levelled against Armstrong are that it is unclear what his relation of contingent necessitation is, and that it is unclear how it is able to necessitate anything.
Gist of Idea
Armstrong has an unclear notion of contingent necessitation, which can't necessitate anything
Source
comment on David M. Armstrong (What is a Law of Nature? [1983]) by Alexander Bird - Nature's Metaphysics 3.1.2
Book Reference
Bird,Alexander: 'Nature's Metaphysics' [OUP 2007], p.47
A Reaction
I suppose someone has to explore the middle ground between the mere contingencies of Humean regularities and the strong necessities of scientific essentialism. The area doesn't, however, look promising.