Using/Import statement in code doesn't see reference of dll just added.

I add DLL's to my projects all the time (in C# and in VB). After employing the IDE to get the library into the project, I just type the normal "using theDLLsNameSpace" (or "Imports the DLLsNameSpace") to begin coding with that library. Nothing surprising there.

The issue I'm having today is that after adding the DLL to the project (successfully and without error) I type this:

using System.Data.SqlClient;
using System.IO;
using System.Windows.Forms;
using theDLLsNameSpace; //The problem is here

namespace XYZ
.
.

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The good-old red underline appears with the following error (on line 4):

The type of namespace name 'theDLLsNameSpace' could not be found (are you missing a using directive or an assembly reference?)

By the way, I compiled the code myself (someone else's original code) in x86. The x86 is just because we've avoided some bugs doing it that way.

Any idea why this is happening or how to fix it?