When adding a domain alias to a domain in Helm, it seems to add correctly. But when I go to the domain in IIS, the alias hasn't been added to the aliased site's host headers. I'm not getting any errors.
This problem occurs if the XML file storing details about Helm's stored sites gets out of synch with IIS.
Go to the webserver that is hosting the domain. Go to the C: drive and you will see a HELM.Config folder. Open this, and delete the IIS.xml file in there.
Now try and add a domain alias. It will take a little longer the first time as the XML file needs to be recreated, but it will fix the problem and will create aliases at normal speed from then onwards..