Fedelity Accounts Department a Disaster

I have two car track contract with fedelity and I have been trying to change my debit order date from Feb 2024 it's now August (Not changing banking details but debit order date). from the last day of the month to the 15. One account was changed around April but the other one has been a struggle. I give them a call every month after every month when they debit my account when they shouldn't and every time they didn't deduct on the on the 15 and each consultant has always maintained that they have now changed and they don't know what happened. Guys what could be so difficult in just changing a debit order date?