Portal for Arcgis web app tile layers not visible

[chailasikasula]

Hi I created Apps for different sites situated in different locations  using Portal for Arcgis .All the Apps seem to be working just fine apart from 1 site were I m getting complaints that some published tile  layers are not visible even when turned on in layers tool. 

What could be causing this problem? I asked that the proxy parameters be added to the LAN settings but I m told the problem still persists.

Please help