No signal bar in codec info view at the beginning of opening a live TV stream

Hi, I run LibreELEC with kodi 17 beta 4 and TVheadend 4.2 on RPi3. Everything works great.But when I open a live TV stream I cannot see the amount of the signal and SNR bars in codec info view for about 10 or 15 min.Then suddenly it fixes the problem automatically. Why isn’t it possible to display Signal and SNR from the beginning? Should I change any configs?