Tag Bad Quality while reading tags from Siemens S7-300

Hello there,

I have successfully connected Siemens S7-300 to the Ignition and Connection status also shows VALID.
After the connection, I added few tags into Ignition and observed correct values. However after some time tags added shows BAD quality and hence read value are showing red. please suggest why its causing.

ignition version: 8.1.1
windows: win 10 pro