The IRS sends a CP23 Notice when it believes you owe more money because you did not pay as much in estimated taxes as you stated on your tax return. This may be because the IRS didn’t apply a payment, the IRS lost a payment, or the IRS applied a payment to a different tax liability.
If you made a payment and the IRS did not apply it to your account, you will need to provide proof that you made the payment. This might include a canceled check, bank/credit card statements, or a receipt from the payment processor.
In some cases, the IRS might not credit your estimated tax payments received if you leave off what it was for. Every payment you send should have the correct tax form (usually 1040 or 1040-ES) and the tax year it’s for.