I think I've found the problem!
On every page of prints.stephen.com, Zenfolio has added the tag:
<meta name="robots" content="noindex,nofollow" />
This is NOT the case on:
http://www.thompsondigitalimage.com/
which has been given as an example of a Zenfolio site which is successful on Google.
In other words, Zenfolio is SPECIFICALLY telling Google to NOT index my site and this is MY site in particular, not all sites.
I think you can imagine that as a paying member, I find this REALLY annoying!