Victor,
It's difficult to determine what goes wrong using the information you provided.
If the button is in a pop-up window, make sure that the window is visible.
If the button is visible, most likely, something happens with the mapping settings. Open the Name Mapping editor and explore the mapping settings of the button and its parent objects in the Mapped Objects section. Perhaps, for some of them, the mapping criteria do not match actual property values. For example, the button's caption or index could be changed. Modify the mapping criteria, if needed.
An alternative approach is to change your test and search for the button with the Page(...).NativeWebObject.Find method. See also Finding Objects on Web Pages.