Vinyl Phone Case Decoration

Both my sister and I have Griffin Survivor iPhone cases as we always drop them (on the floor or in water) but they look boring and we wanted to jazz them up a bit. I used the silver vinyl from the Cricut Explore starter set and chose a Little Mermaid text design for mine from a Cricut cartridge, and my sister had a Tinkerbell silhouette I uploaded myself. 

The designs stuck well for the first day but then started moving and letters started coming off mine, and the wand on Laura's Tinkerbell moved too. I think it's due to the material used for the phone case as it's not got a completely smooth surface. I think a stronger vinyl may work better or if I used the vinyl as a stencil and then painted through it. Vinyl sticks fine to other phone cases, I just think our cases that have a problem with the vinyl.