First: Because some waves under a certain frequency in a certain material are eliminated / cannot propagate. Try with a higher frequency and see if it travels better.
Second: The material surface needs to be stretched enough with each pulse to draw enough power from phone-vibration so the phonons will have more and more amplitude. Ground:does not stretch. Bed surface: streches.
Third: you may not have put your ear to ground properly. Have you?
Fourth: Your body can not be coupled with the ground but can couple with bed-vibrations in terms of vibration. Because your body's natural-vibration-frequency is not equal to ground materials' but is equal to bed.