Hi Arghya,
Can you please confirm when this issue occur. Because we has similar issue and we found out that users actual session was expired and from that point UWL was not pulling data. When you hit F5 or refresh on browser screen, it will refresh Portal browser session without prompting you to enter user id/password. If user does not close current browser then issue exist. If user closes his browser and starts new browser session then everything works as expected.
So to confirm, when you run into this issue, clear browser cache & cookies, close all browsers, open new browser and test. Please let us the results.
Bhavin