Payment plan setup but still receiving letters about late payment

Hi, I set up a payment plan in January on my Self-Assessment, and it has been working successfully. Payments are being made and received monthly. Yesterday I received a letter telling me that I owe £330 and it is late, and must be paid. This is obviously incorrect because I have a payment plan. So why am I being told that it must be paid now all at once? I have tried calling over the phone, repeatedly, but get useless information and direction to irrelevant information I've already read. It's incredibly frustrating and I can't help but think a complete waste of time. Please could you advise?

Hi

If the amount wasn't included in your previous plan, it may be an underpayment that wasn't previously on your record.
For example, there may have been an amendment to your return after a clarification of your income or other figures, resulting in a larger amount due.
To give specific advice, we would need to review your record - contact us by webchat via Self Assessment: general enquiries to allow us to view your statement and returns.

Thank you