If the headset icon in SteamVR window is pulsing green, or the screen on the headset is displaying light grey, it means that the headset is losing tracking. Possible solutions you can try are listed below:
Make sure that the headset is in the play area, which means that it is in the view of at least one of the base stations.
Check the base station and make sure they are working normally with the green lights on.
Make sure the base stations are set up at the correct height (at least 6.5 feet) and angle (30-45 degrees down). Also make sure the base stations are mounted firmly without shaking when connected to power.
Make sure all cables are connected firmly, especially the HDMI cables. Unplug and plug back all the cables might help.
Check if there are reflective surfaces (e.g. windows, mirrors, glossy walls, etc.) that are interfering tracking and put covers on them.
The front camera sometimes could also cause tracking issues. Try lowering the camera bandwidth or disabling it.