Working with resources is so much easier in .NET 2.0 but things do occasionally go awry. Most of the time the problem is really easy to fix.
As discussed in another post, Visual Studio 2005 does a lot of work behind the scenes to surface your resources as strongly-typed objects; if you're not careful with your .resx files, however, you might end up in a situation where your .resx files aren't being compiled for you. As a result, you lose Intellisense for your resources, the ResourceManager may end falling back to your default resource file when it shouldn't, or your default resource file might not load at all. Copying and renaming Visual Source Safe-controlled .resx files is one little culprit that occasionally brings everything to a halt.
It's important to remember Visual Studio doesn't just "do" things for you--it must be told what to do and frequently relies on stand-alone tools included with .NET or sitting outside of the VS shell. A good example of this can be seen by inspecting the property sheet (in Visual Studio) for one of your .resx files.
When you create a new .resx file Visual Studio does all the right things by setting the Build Action to "Embedded Resource" and setting the Copy to Output Directory as "Do not copy". Just as importantly, Visual Studio also sets the Custom Tool property as "ResXFileCodeGenerator" and this particular setting can occasionally get stripped away when you're renaming or moving resource files. If in doubt and your resources are not being made available to your application, check this property; if it's not set, set it to "ResXFileCodeGenerator".
If the Custom Tool property on your .resx files is set correctly, Visual Studio will help you out by running the specified tool for you every so often to ensure your resources are available programmatically (this can actually be a pain in the neck sometimes so I recommend using a tool like Resourcer to edit your resource files...). If this isn't happening, you can simply right-click on your .resx file in Visual Studio and select Run Custom Tool.