Alias issue on a Shared IP (Not Default)

Hi there,
I have a slight issue with an alias server against a non-default Shared IP address.

Against this IP, I have the following:

domain1 (this is the default website for the IP address)
domain2
domain3

These all work correctly as you would expect.

I have tried to add an alias to domain2 as follows

I have updated the dns records to point to the IP address.
When I access domain2a in a browser, it goes to domain1 instead of domain2.

What have I missed?

Thanks,
Craig