Issues with activating corporation tax

Hi HMRC team, I've requested activation code for corporation tax as posted via mail, however it hasn't worked for me even though I've entered it within the activation window. I need to file for my companies account soon, and this isn't the first time it has occurred. Please help! Thanks

Hi 
Please refer to our website here:
Technical support with HMRC online services
For help with this.
Thank you.