Mesh Snag - Mesh does not follow armature.

I made an armature for a leg. I mirrored the leg and the armature. On the original leg and armature, with “With Automatic Weights” applied, when I rotate the armature the mesh follows it like it should.
But on the mirrored armature (also using “With Automatic Weights”) the mesh snags as in the attached screenshot. I am using v. 2.65. This is frustrating :spin:
I will appreciate any help.

I got the lazy vertices to move with the rest. Had to go to weight paint and redden the lazy area. The strange thing is that this is a mirror of the other side where I did not need to mess with weight painting. Oh well.