solving a problem that affects a specific device is difficult

does the device have a problem with this site or our demo site, both of which use Mobile Kit Pro?

have you tried the other skin included in mobile kit, does the same problem happen there?

have you tried clearing the cache on the device web browser?

possibly could also be a carrier issue if the carrier puts a proxy between your device and the web and caches content on the proxy, I know kindle does that with the silk browser but not sure if other carriers do it, but it could be a factor.

is the lumia on the same carrier?

I suppose its possible that a carrier may use different settings for different devices according to what the hardware supports.

It is strange, have you googled about web problems in general for that device to see if others have any weird web or network problems?

you get the same result no matter if it uses wifi or cellular network and using different wifi hotspots?

if it is a device cache issue, I would think making a copy of the skin with a different name and changing to that copy would get around the previous skin being cached