Hi Shaun, when you can't join things that means there is a gap between them of more than the maximum Join tolerance which is 0.005 units. Right now the Network command can sometimes generate a surface that is slightly over that distance limit from its input curve - that's because of a mechanism that tries to make a lighter weight output surface to handle dense networks better.

In v3 I should be able to tighten up the fitting of just the outer edges of the Network to avoid this from happening.

But when it does happen, you can still get things joined by scaling your objects down and then doing the join and then scaling back up again - the full steps are described here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3939.2

That's what I did in this case to build the attached joined solid version of your object.

- Michael