It's cuz you're not doing it correctly.
Yes, you're supposed to depress "vol down & power" but once you see LG logo, let go of both buttons and then immediately hold down only the "power" button.
After a few secs, a white factory reset screen will appear. Ignore the warning and press power button. I know, its kinda scary at first, but then phone will load into recovery.
Fully power off phone and try the following one should work:
1)with phone powered off hold volume down+power and when lg logo pops up let go and quickly press again
2)with phone powered off hold volume down+power and when lg logo pops up let go and hold volume down+up don't touch power
This will lead to factory reset menu press power button 2 or three times and phone will boot into custom recovery
Sent from my potato powered LG G3 D800