When you import a file into HubSpot, you can download a list of any errors that occurred during the import.
You may find an error with the error reason:
Header and row sizes don't match
This error means that you have information in some columns where there is no header indicating the field where those values should be stored. This can happen if your file once had additional columns containing information that were not deleted properly before importing the file.
If you are opening the file in a spreadsheet editor (such as Excel) the extra columns may look blank, but if you open the file in a text editor (such as notepad), you should see a number of superfluous commas.
To correct this, you can search (Ctrl or Cmd + F) and replace all of those commas with one comma, then save and re-import your file.