Thanks for your reply!
As it turns out, I was using jrb tool gwusers to create these accounts (intending to do users en masse). If I manually create the user with C1, it works correctly, but using gwusers.exe, though the object shows up and appears to be all good, I'm unable to get it into the Global Address book.
But, as you say, perhaps if I use the tool, then do a gwcheck, maybe that would fix whatever is missing? I'll try that and report back.