Voluntary payment made - State pension age reached

I was told earlier this year to complete my 2023-24 tax return to enable my NI shortfall for that year to be calculated. I paid that shortfall on 28th May and was assured that this would complete my NI record and entitle me to receive the full state pension. I reached state pension age last month and my first payment indicates that I am not in receipt of the maximum. I checked my NI record and the record for 2023-24 still does not show the year as being complete. I contacted HMRC on Monday 8th July and an agent there confirmed the payment had not been processed. He (apparently) filled out an online form and assured me that after two working days, the payment would have been processed, to wait a further day by which time my NI record would have been updated. 9 working days later and the payment has still not been processed , so therefore DWP cannot recalculate my state pension entitlement. I am now at a bit of a loss as to what to do next. When will the payment be processed and when can I expect to receive the full amount plus the shortfall from my first pension payment?

Hi Belly Belly Bell,
Unfortunately, without having access to your National Insurance records, we are unable to advise you why the payment has not been allocated.
You would need to contact the National Insurance helpline to check why the payment has not been allocated following your call on 8 July 2024. Once the payment has been allocated, the pension service will recalculate your pension entitlement.
We do not know what their timescales are, you would need to contact them for further information regarding the pension payments. 
Thank you.