I believe I found out what the problem was. I wasn't thinking about the vertical orientation of the screen on the phone or tablet, and the background image wasn't designed as a vertical picture. That's why it's not stretching the length of the screen, only the width. BUT if you turn the phone to the side where the screen is longer (landscape), it works. I've decided to use a variant version on both phone and tablet versions to fix the problem. Thank you for your help.