I think this is buggy behavior in Visual Studio. Unfortunately I don't know of a sure-fire way to fix it. Some things you might try:
- Close and reopen the project. Sometimes this works, particularly if you have images that don't seem to display properly. (Fairly often when I add an image to a control, the image doesn't appear. Closing and reopening often fixes that problem.)
- Select the MenuStrip, press [Ctrl]+X to cut it to the clipboard, and press [Ctrl]+V to paste it back.
- Start a new project and use copy and paste to copy the MenuStrip to the new project.
- Start over.
If you open the designer-generated file behind the form, there's a chance you can figure out what is wrong, but that can be tricky so I wouldn't recommend it unless you're pretty sure you can't salvage the project and you just want to poke around in there.
Sorry I don't have a better answer.