This issue is most commonly seen on sites that are not optimized for mobile screens. Typically, either the placement is too big or too small in relation to the viewport. This is most commonly seen with interstitial formats where the placement appears outside of the viewport area so a workaround has been used to force the interstitial into the centre.
Interstitial banners scale up in relation to the size of the device viewport. However some sites don't use recognised best practices to handled responsive site layout on mobile views. If this is the case it is highly likely that the incorrect placement size will be used resulting in an ad that, for example, doesn't completely fill the screen.
How To Fix It
Ideally, websites should use responsive (or adaptive) layouts that adjust themselves correctly to the user's device. In this case, the placement appearance on mobile views needs to be handled using standardized, widely accepted methods, as opposed to workarounds.
If this isn't an option then our developer team can implement a workaround for your channel. Please reach out to our support team so they can resolve the issue.