HMRC places restrictions on when you can file an amended CT600 return online:

• You can only file an amended return within 12 months of the filing date.
• In the case of a return for the wrong period, you can only file within 12 months of what would be the filing date if the period for which you made the return was an accounting period.
• If the original return was late, the time limit can't be extended.
• If the original return was more than 12 months late, it can't be amended.

If you believe you've made the amendment within 12 months of the company's filing date, call the Online Services help desk on 0300 200 3600.