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There may be a number of reasons for DMSS HD to not be working, so we will go over the notable and common reasons here.

The first thing you may want to do is completely close and re open the app. This can fix many issues and is an easy step to try first in case it does solve your issues.
Make sure your device is connected to WiFi if possible. It is possible to run DMSS HD without WiFi but it may be laggy and may not show the full image quality, if you are encountering these issues, connecting to WiFi may solve them.
Similar to the first issue, if you are connected to WiFi but nothing is loading, you may need to reset your router.