Yaobing, I have only seen the problem once, and that was on version MC20.0.126. But I don't run TV from a Client all that often, as many people do. That may be the common element.
I suspect that the one time it did happen, it was because I had been running TV on a client quite a lot, starting and stopping various channel, switching channels, etc. That is, forcing the server to allocate tuners many times. Since I have four tuners, it took a while for the problem to appear, but when it did, I had no tuners listed in the "Show Status" dialogue on the server.
All I had to do to fix the problem was go into the Television "Manage Devices" dialogue on the server, and back out again. I don't think I even hit OK to exit, just cancelled out. The tuners then all reappeared in the "Show Status" dialogue, and TV started working again on the Client.
Interestingly, I have a Webcam on my Client PC which appears as an available analog Television device on the Client. However, this regularly disappears from the "Show Status" dialogue, until I go into the "Manage Devices" dialogue, then it reappears. If I close MC and reopen it, the Webcam has gone again, and doesn't reappear even if I allow lots of time, but does as soon as I go into the "Manage Devices" dialogue. That behaviour did start some time in the past, as the Webcam used to always display, I think. It could have been around MC20.0.115, but I don't really know.