Look at form 8889. To allow a deduction of $6300, you would have to have family HDHP, and if you are under age 55, not have more than $2000 of other contributions (including payroll contributions from you or your employer). The form 8889 for your amended return should have "family" checked on line 1, line 2 should be $6300 according to your story, line 3 should say $8300, and line 13 should say $6300.
Then, the amount from line 13 of form 8889 should be repeated on line 13 of schedule 1.
Also, did you e-file your amended return with Turbotax? If so, that should not allow a computation error. If you prepared your return by hand and mailed your return, it's possible you made an error somewhere else on the form.
