VS 2010 Switching between forms

I have a program where I display a master form on which I then display another form which only fills part of the master form making it look like a single form. This has worked fine for me in the rest of the program but now I have a form where I select a different sub form without returning to the master. This also works fine till I want to go back to a sub form I have already used then I get the following error.

Form that is already visible cannot be displayed as a modal dialog box. Set the form's visible property to false before calling showDialog.

I am switching between forms by using

FORM A
me.close()
FormB.showdialog()

FORM B
me.close()
FORMA.showdialog()

I thought the me.close() closed the original form so I should be able to re-show it.