If your background image is incorrectly proportioned (e.g. a landscape image instead of portrait), then for templates which utilise a full-screen background image it is possible that the image will be scaled down in such a way that it only displays on the top half of the screen:
This will also cause the text box to appear blank, as the default text colour for these templates is white.
To prevent this from happening, crop your image to a portrait aspect ratio (the ideal size is 750 x 1624 pixels; an aspect ratio of approximately 6:13)
For more info on preparing images for your mobile landing pages, read this article