Overall, I have had good results with MB2, but there have been some inconsistencies. My suggestions for giving yourself the best possible results. I scrub my MB2 with soap and water, using the scrubby side of a kitchen sponge, rinse thoroughly, and air dry. I then wipe it down with rubbing alcohol on a paper towel. This has generally yielded good results, though there have been one or two that did not take well even after going through this. I found on the MB's that did not take well, once I removed the lifted decal, and applied a second one (MagicYourBand always provides two copies of each decal) that the second one always sticks.
Do be careful not to touch the adhesive part of the decal any more than absolutely necessary. Make sure your hands are clean as well. Oils from your skin could certainly keep the decal from adhering well. The other thing that I've thought about, but not tried / tested, is using a bit of tape to clean the band with. I thought about this because when ever I have had a decal that lifted, putting a second decal on afterwards has always worked, so I wondered if there might be something on the surface of the MB which sometimes survives my cleaning process, but then gets stuck to the adhesive on the decal. Perhaps a bit of duct tape applied to the bands, and pulled back off might help to remove anything on the surface, and give the decals a better chance of adhering.