Hi,
Your port may be contaminated with dirt or debris, so you can try cleaning it out.
First go to YouTube and find videos on how to clean charging port for your device.
Check your cable end too, as this may be dirty as well.
It might be a good idea to try another cable as yours may have wires that are damaged and not making connection in certain angles.
If carefully cleaning the port does not fix the problem then there may be considerable wear on the port and you may have to replace it with a new port.
Hope this helps