14 CFR § 61.49

An applicant for a knowledge or practical test who fails that test may reapply for the test only after the applicant has received:

The necessary training from an authorized instructor who has determined that the applicant is proficient to pass the test; and

An endorsement from an authorized instructor who gave the applicant the additional training.

An applicant for a flight instructor certificate with an airplane category rating or, for a flight instructor certificate with a glider category rating, who has failed the practical test due to deficiencies in instructional proficiency on stall awareness, spin entry, spins, or spin recovery must:

Comply with the requirements of paragraph (a) of this section before being retested;

Bring an aircraft to the retest that is of the appropriate aircraft category for the rating sought and is certificated for spins; and

Demonstrate satisfactory instructional proficiency on stall awareness, spin entry, spins, and spin recovery to an examiner during the retest.