The IRS sends Notice CP24 or Notice CP24E (CP25) to tell you their records of your payments do not match their records & they are adjusting your account.
Respond to Notice CP24
Look carefully at your tax return and the IRS notice to see any discrepancy. Find your checks or payments to the IRS and see if they match with the IRS numbers.
Sometimes, the IRS has applied your payments to a different time period than what you intended. You'll have to get your account transcripts for ALL time periods to see where the IRS placed your money.