When doing a Supporter Import, if you're electing to import supporters into a group (or multiple groups), then you must make sure that the supporters aren't already part of the group(s) that you're trying to place them into.
For example, you already have supporter firstname.lastname@example.org in your supporter list and he's currently assigned to group XYZ. If you're doing an import which includes email@example.com, and the import specifies that he should be placed into group XYZ, then the import will error on that row (because the supporter is already in that group) which causes the import to skip over the row entirely.
To get around this, here's a work-around.
(Internal reference: SCT-512)