Try other HDMI ports. Since I don't have all the details here the most common is it's on or not on some HDMI ARC port and there cable is or is not ARC ready or there is some incompatibility.
It's a common problem which has some dozen solutions including changing ports, cables, set top boxes and firmware updates in the TV.
Is it the TV or Comcast X1 Xfinity?
LG 55LB6300 Smart TV
When streaming Amazon or Netflix shows, an alert window pops up randomly telling me my set top box isn't connected. Well, yes it is connected, through an HDMI port. This only happens when streaming video from these two sources, and not with any real-time programs.
I recently upgraded my cable service from Comcast with their X1 Xfinity box -- also connected to the TV via a HDMI port -- and wonder if this system is causing a conflict.
Of course, I'll post this question with Xfinity user groups as well.