There can be several reasons behind Lg AC not cooling problem, some of which are technical faults and some are non- technical.
If the Lg AC does not cool, the air filter might be clogged. In such case, air cannot flow through properly which greatly reduces the cooling capacity of the AC. Either clean the air filter or replace it as needed.
If the Lg AC is not cold enough the condenser coils may be dirty and loaded with debris. In an AC, the condenser is like a radiator that dissipates heat (which was removed from the air). The condenser coil must be clean to dissipate heat outside. As the coils get dirtier the AC becomes less efficient, which makes it work harder to cool the room. If the coils are dirty enough, the unit will never be cold enough and will not be able to cool the room
The coolant circulates through the evaporator coils and cools the air flowing over it and eventually sends cool air into the room. So when the level of this coolant becomes insufficient, the Lg AC fails to cool. At this stage call a PRO for coolant top up and refill.
A compressor is the heart of an air conditioning unit which is responsible for cooling your room. If the Lg AC does not cool, the compressor might be defective. The compressor is a motor which compresses the refrigerant or coolant and circulates the refrigerant through the evaporator and condenser coils. If the compressor goes bad the cooling cycle does not start and the Lg AC fails to cool.
A thermostat is a sensor that senses the room temperature and hints the compressor to start or stop the cooling cycle. If the thermostat is defective the Lg AC does not cool or may cool continuously.
An AC has an optimum operating temperature (surrounding temperature) at which it can work most effectively. If the temperature shoots up beyond the optimum temperature limit, the efficiency of the AC goes down. Sometime it is so hot outside that a stage is reached where the compressor stops working completely.
A common mistake people make is that they buy an AC based on their budget and not based on the requirement of the room. A small AC in a huge room will always find hard to cool the room in peak summers. The size of the AC should be according to the size of the room.