Manual LG G2 root directions below. It may look like a lot but that is only because step 2 lists commands for every single model/firmware. 

Manual root is useful if the ioroot scripts fail. Usually the script will fail due to lack of support for your specific carrier... 
or you are on a new firmware update. 
Examples: 
#1 You are on a D802 but your carrier is a local German carrier which is not supported by the script. Just because your carrier
is not supported does NOT mean there isn't a method for root. Look at the other D802 manual root steps(step #2 specifically) and they will most likely
work for your carrier as well. D802/D805/D806... this method should apply. Currently with D802/5/6 there is one method for all.. no matter the carrier.
This may change when official KitKat is released... at that point there will most likely be TWO methods.. one for JB(4.2.2) the other for KK(4.4).

#2 You are on an F320L KitKat stock rom, unrooted. Currently only F320L 20B is supported by the root script.. but you just did an OTA for 20S. 
Try the manual steps for the 20B method and root will most likely work. 

These examples are not bulletproof but should work in most situations when the automated scripts do not work. 

Want your carrier added to the next version of ioroot? 4 questions need answering(you can post in the official thread)...
...which ioroot version? which phone model? which firmware version? 
and if you run: adb shell "getprop ro.build.target_operator" what is the operator it gives you?


Now for the actual manual root steps...
see the "Manual Root Method" in this post: http://forum.xda-developers.com/showpost.php?p=48709232&postcount=869