UEFI BIOS

If I leave the power connected to the computer all the time it boots every time. If I disconnect the computer & later reconnect it to power, a boot attempt gives you “No bootable device found”. Going into the BIOS and saving it (none of my settings have changed) fixes the problem until I leave it unpowered. This happens with more than one computer. One is an ASUS P8Z68-V Pro/Gen3 & the other is an MSI Z87M-G43. Why?