Head to your My Imports page, then look to the column that says Not Imported.
Click on the number under Not Imported and you'll see a breakdown of the reasons they were missed. You can then take it a step further and export a list of those contacts along with the associated reason they failed to import. To do this, click the link to Download all Not Imported contacts.
Please keep in mind that this information is only available for download for 7 days.