More often than not, this is purely a propagation issue that needs to be given more time to complete.
To be sure that the signature has failed, check your Logs located under Settings > Logs in the left hand menu.
If you can see an error message for the affected user mentioning signature, then it has indeed failed.
If you cannot understand the error message, please get in contact with our support team at support.cloudm.co
If there is no error message and a lot of time has passed, be sure to check that the email address affected is the primary email address.
Check that they’re also part of the OU that’s being changed
Make sure that they have email signature enabled for the OU in question.
If it is inheriting, ensure it is enabled at the parent OU.