Taken from another post on here (Why is my LG G4 not charging properly?):
There is a series of steps you can take to attempt to fix this issue.
1) Press and hold the power key for 10 seconds to reboot the phone.
2) If that does not work, try using a different outlet or a different charging cable.
3) Next, you can try performing a hard reset on the phone. (Make sure to back up all your files as they may be lost in this process)
Turn the power off.
Press and hold the Power key and Volume Down key on the back of the phone.
Release the Power key when the LG logo is displayed, then press and hold the Power key again.
Release all keys when the Factory hard reset screen is displayed.
Press the Power key to continue or any of the Volume keys to cancel this process.
Press the Power key one more time to confirm the reset.
For other issues related to charging the LG G4, please refer to this Troubleshooting Guide: LG Phone Troubleshooting
Other than the already mentioned tips above, you may want to try cleaning your charger port carefully with a pin or sim ejector tool. Usually shining a torch into the charging port will reveal years of dust and pocket lint build up deep in the bottom of the charging port.
Hope this helps :)