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.io
No Error - 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 OU - Check that they are also part of the OU that is being changed
Enable Signature - 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.
Sync Email Signatures - If you have checked and ensured the above, you can try to force the email signatures to sync manually at either Root or any other OU you choose. Navigate to the OU of your choice, select Actions, click "Sync Email Signatures" and this will force a signature sync. This often better than doing a domain sync. Note: See this article.