I was dancing last night and my LW and I danced a couple of VW's with no trouble, but then I danced with another lady who started the underturning. I started trying to determine what was happening, and I discovered that she was stepping (on her 2nd) step outward instead of forward.
Seriously, it happens sometimes. Friday, I'm going to watch some people doing VW and see if I see anyone either taking that second step sideways and back towards wall, as Larinda described, or letting their cross degenerate into a spiral turn, which is what I think happens sometimes. But either way, it's not that big a deal. I just let it be a 3/8 instead of a 1/2, and I follow it up with a right turn. I've gotten to dislike doing a bunch of left turns in a row anyhow -- it's a dance, not a track meet.