I am attempting to import a csv file that is both regular comma delimited and I have also tried one saved as CSV UTF-8 (Comma delimited). Each time I go through the steps, I get through the match fields correctly. When I get to Step 3 Preview, I am told that my total number rows have errors and won't be imported:
Without seeing some of the actual file that is being imported, it is hard to say for sure. But my guess is that birth date field is an actual date field? And if so, it needs to be formatted as a date and on the first import screen you have to tell Civi how it is imported.
The error file that you can click and download will give you more information, as it will tell you what field(s) are the problem.