To see why not all items have been imported, click on the message number.
The "Message" will open, with more information about the message.
Scroll down to "Validation logs", for more information about the validation errors.
Under "Description" it´s possible to see an explanation of the validation error.
"Properties" will show any property affected and "Location" will list the items that have not been imported.
In this example the barcode checksum validation have found incorrect barcodes. Could be that they are added to the wrong column in the template, have the wrong barcode type.
These items needs to be corrected and uploaded again. Could be in the same file that has previously been uploaded, with the items already been imported, or in a new file.
More information:
How to see in "Messages" that not all items have been imported?