Thank you for your post.
Without seeing your file or script, it is a little difficult to determine where the problem lies.
If the last record in the single table is blank, then you would create a new record and replace the information in the fields. Since no information exists, then nothing is entered with the Set Field steps.
Are you doing any kind of evaluating before the new record is created? That is, have you tried
If [ not IsEmpty ( <key field> ) ]
This checks to see if any information exists in the key field. If so, then you can add the record and replace the information.
Any other information you can provide may be helpful in determining the possible cause(s).
I seemed to have fixed it. I think the prob was in the checking of an existing record done improperly.