This is a common issue because the Pi needs to read the EDID data from the TV at boot time, but some TV’s don’t provide this information if the TV is in standby while some do. The solution is to save and enable use of an edid.dat file:
http://kodi.wiki/view/Raspberry_Pi/FAQ#TV_is_not_detected_unless_powered_on_first
The OSMC addon Pi settings module provides a way to automate this.