Defective Council Tax Reduction Claim

I have a client who made a CTR claim back in August 2019 which was defective i.e incomplete and did not get processed - he had ongoing Good Cause for his failure to respond to the requests for information from the council - due to a health condition.  I made another application for CTR with him in May 2021 and requested that they would consider backdating to the date of the initial Defective Claim - they gave me three months only backdate. Decision was made on 04.08.21

Am I still able to request a reconsideration of this 04.08.21 decision as I have only just had the medical evidence to support the initial failure of the defective claim?

Thanks