Sometimes moi just refuses to join 2 surfaces for a couple of reasons.

1) Try saving and then reloading the mesh. Often joins/unions/etc will work after this

2) Examine the profile curves you used for the sweep, and check for overlapping points ( Click show points ). Overlapping points will show up as a red box. Just select and delete one. These overlapping points will generate sweep curves just fine, but cause problems with joining the resulting surfaces if they occur at the edge which you are trying to join another curve to.

Perhaps moi should pop up a notice when sweeping a curve containing overlapping points? This has bitten me a few times. You can join the swept curve to other swept curves at the edges that does not contain a overlapping point.

Also, I notice trimming circles/ellipses close to the quad points leads to overlapping points in a lot of cases.