Problem with text box in masterslide

Hello,

I have a problem that is driving me crazy.

I create a textbox in the master slides, then apply that master to one of the pages.

I then shift click the textbox to write some stuff in it. If I move the textbox in the master slide, the textbox in the page moves too.

But after a while, I don't know what happens but moving the textbox in the masterslide doesn't move the text box anymore!

Another textbox in the page gets created AND it has the original textbox from the masterslide.

Does anyone know why this happens? Thank you in advanced.