Device Syncs at Office not at Home

I'm running Mobility 2.1, no security policies are enforced, and we perform as generic connection to our Mobility server as possible when we connect a mobile device. I also have both internal and external DNS references that point to our Mobility server. Over time I go through phases where iOS users experience issues where their devices quit syncing when they are not connected to our office WiFi. Again, we don't have any security policies that would restrict this behavior. Case in point I just spoke to a user who's iPad does not sync at home but does sync when she brings it to the office, meanwhile the same user's iPhone syncs without any problem from both locations. Both devices have identical settings, but I have not yet verified iOS versions, but imagine that they are both up to date.

Judging from the behavior I would almost say that the iDevice's DNS cache gets stuck with our internal address, so when they take the device home it quits working. In the past I have resolved this issue by first having the person try rebooting their device and if that doesn't work delete and recreate their local account, but that seems extreme. I'm just wondering how wide-spread this issue may be and if there are any other easier fixes. Thanks!