does the guy have javascript turned on? is he using charles or some other proxy system to route his data and just not forward it to your metrics collection server. Are you trying to send secure data to your non-secure metrics server or visa-versa? have you gone to your metrics server and typed in /optout.html as the url to get your opt-in/opt-out capabilities:
To opt-out of aggregation and analysis of data about your visit to this site, it is necessary to install a cookie on your browser. This cookie identifies that you have opted-out.
If you delete the opt-out cookie, or if you change computers or Web browsers, you will need to opt-out again.
Opt-out - Exclude me from visitor session aggregation and analysis (install the xxx.com opt-out cookie) - Click Here
Opt-in - Include me in visitor session aggregation and analysis (do not install the xxx.com opt-out cookie) - Click Here
where xxx is your company/domain the cookie in that domain is called "omniture_optout" and it will keep metrics from being collected.
if you can access their system see if you can grab a network tab output from developer tools to see what is supposedly being sent. maybe the report suite names are wrong or the requests are being aborted before they complete (some browser settings will let you do this)
Does the customers companies firewall block out access to whatever your metrics server address is? maybe they have an ip blocker that blocks the metrics collector (if they can see the optout.html file then it's not blocked.)
maybe they access your site through some fake site shell that blocks analytics access or throws it someplace else. Again looking at the actual network request to see where it's going could help.
look at http-fox plugin for firefox and see if it shows anything when the analytics requests go.
that's all the non-obvious ways I know of to block requests.
