There are several different reasons banding or uneven lifting may occur. If you had permanent/box dye put on your hair at one time, this can prevent it from lifting the same as the rest of your hair. If parts of your hair had previously been lightened while others hadn’t, this can also cause the color to lighten unevenly and cause banding. Basically, any unevenness in your base may result in an uneven lift too.
If your lengths are already pre-lightened, you need to be careful not to overlap the bleach onto the lightened parts or you may experience banding or breakage. If you have overlapped by mistake, you can always wipe back the mixture during processing and you should be fine.