IIS 7.5 ignoring HttpCompression settings in web.config

I'm trying to enable dynamic compression for the mime type application/json.

In applicationHost.config, I've made the following change:

<section name="httpCompression" allowDefinition="AppHostOnly" overrideModeDefault="Allow" />

I also tried unlocking the section with the following command:

appcmd unlock config /section:system.webserver/httpcompression

My web.config settings (same as applicationHost.config but with additional mimetype):

    <httpCompression directory="%SystemDrive%\inetpub\temp\IIS Temporary Compressed Files">
        <scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll" />
        <staticTypes>
            <add mimeType="text/*" enabled="true" />
            <add mimeType="message/*" enabled="true" />
            <add mimeType="application/x-javascript" enabled="true" />
            <add mimeType="application/atom+xml" enabled="true" />
            <add mimeType="application/xaml+xml" enabled="true" />
            <add mimeType="*/*" enabled="false" />
        </staticTypes>
        <dynamicTypes>
            <add mimeType="text/*" enabled="true" />
            <add mimeType="message/*" enabled="true" />
            <add mimeType="application/x-javascript" enabled="true" />
            <add mimeType="application/json" enabled="true" />
            <add mimeType="*/*" enabled="false" />
        </dynamicTypes>
    </httpCompression>

But the response is not being gzipped. I know the settings are correct as it works if I add the mimetype directly to applicationHost.config.

I've enabled Failed Request Tracing and no errors are produced.