Samashy on "Theme always switch back to the old"

Hi,

I activated a new theme for my blog and customized my Posts, pages and widgets. But after a few hours or sometimes minutes the theme switched back to the old theme (WordPress Default By Michael Heilemann Version 1.6). At the first switched I thought the new theme has a bug. That's why I tried other new themes of different authors... every theme switched back to the old.

Do you have any idea what I can do to fix the Problem?

Greets,
Samashy (Blog2.samashy.de)