The feed system will stop if it finds 10 invalids in a row, this can be the first 10, or even 10 midway through a file.
A key give away will be Row Count, which suggests the amount of records the file has looked at.
A common reason you might want to consider:
Does the CSV File contents match the order in the Selected Fields?
The summary report shown in the screenshot above has a downloadable file that you can retrieve "report" which will give you per lead advice when it comes to invalid leads.
This may help you come to a conclusion on whether it's a common and easily amended issue or if each lead needs to be looked at on a case by case basis.