I believe you will need the resx file as well as the form (vb, cs).
Click the 'Show All Files' button in the Toolbar and check to see if
the form file has a little plus beside it, with a resx file under it when
expanded. If it doesn't, you will have to make sure that yourForm.resx
is in the same folder, and has the same name as your form.
I don't know for sure if this is the cause, but it could very well be.