Master Page suddenly moves

For my first assignment using LiveCycle ES2, I have to make amendments to a complex multi-page form.  If I select a field object or subform and make any changes to a Layout property (for example, width), the first Master Page moves suddenly several cm right and downwards, destroying the registration and alignment of the form.

Any ideas why it happens, and (more importantly) what to do about it - either how to stop that behaviour, or how to recover afterwards?

Thanks for any assistance,

Dave