Variants without parent products will not be imported.
There are two solutions here:
Solution 1: Add the parent to the original spreadsheet by updating your products.
Note that this assumes you are adding variants via uploading another spreadsheet. If this is the case you will need to go back and update the original spreadsheet so that it includes the parent(s).
Solution 2: Add the parent(s) along with the variant(s) to the spreadsheet that you're using to add variants to your products. However, be sure to add the parent(s) before the variants in the spreadsheet.
In the example below Items 2 and 3 are variants of Item 1 and that Item 1 is listed in the spreadsheet before Items 2 and 3: