IIS6 Compression only working over HTTPS - asp.net

Question

We have recently enabled compression in IIS6 for our ASP.NET applications (some 2.0 some 4.0) and have found that GZip is only being performed when the request is made over HTTPS.
This problem is being seen on all servers in our web farm. I have also checked that the same problem exists regardless of the browser used.
I've searched for over an hour and can't find anything on this, so hoping someone may be able to help. Examples follow
HTTP Request:
GET /about.htm HTTP/1.1
Host: www.website.com
Connection: keep-alive
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/29.0.1547.76 Safari/537.36
Referer: http://www.google.co.uk
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
HTTP Response (not GZipping):
HTTP/1.1 200 OK
Cache-Control: private
Content-Length: 65568
Content-Type: text/html; charset=utf-8
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
Date: Thu, 26 Sep 2013 09:26:46 GMT
HTTPS Request:
GET /about.htm HTTP/1.1
Host: www.website.com
Connection: keep-alive
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/29.0.1547.76 Safari/537.36
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
HTTPS Response (GZip working):
HTTP/1.1 200 OK
Cache-Control: private
Date: Thu, 26 Sep 2013 09:29:39 GMT
Content-Type: text/html; charset=utf-8
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
Content-Encoding: gzip
Vary: Accept-Encoding
Transfer-Encoding: chunked
UPDATE
I've run a request trace in IIS - the offending requests have the following line
IISCompression DYNAMIC_COMPRESSION_NOT_SUCCESS "NO_MATCHING_SCHEME"