How many are you trying to import (I think I've bulk imported circa 170-180 in a hit), and have you used the sample csv to make sure you have the correct columns? Have you fully populated those columns (although I know it will allow missing data even from mandatory fields, then default to something when you first edit the client).
Also check your characters, things like apostrophes can cause issues (eg O'Something), as there are multiple formats, and you need to check them all to be the simplest one.