Replies creating new requests

I have noticed that replies to SupportCenter emails are generating new support requests rather than being associated with the original support request.

If UserA emails support@mydomain.com a new support request is created. I then view that Request in SupportCenter, click the Reply button on the Request detail page in SupportCenter and send USERA a question, say asking for more information. When USERA then replies to my email, sending me the info i have asked for, SupportCenter creates a *NEW* support request rather than associating that email with the original support request.

I have checked that the reference number in the subject line of the emails matches that of the original support request, and i have asked the user how they are reply. They are just hitting reply in Outlook.

Any ideas why this might be happening ?

Olly