You probably have rows in the Excel spreadsheet that have NULL (empty)
values in the Primary Key field of the table you are trying to import into.
Also, if the table you are trying to import into has a multi-field index
that has a "Unique" index, then Access will not be able to import two
records from that spreadsheet with the same values in those fields.
I would try:
Instead of importing from Excel, try linking to the Excel spreadsheet. This
should eliminate the import errors, and then you can use queries to look for
the situations mentioned above. You might even be able to identify the rows
that are not being imported (use the unmatched query wizard if you don't
know how to do this).
HTH
Dale