There are basically 3 faults in this case.
First is due to low pressure of the refrigerant inside the air conditioning unit. Refrigerants are basically gases like R32,R22, R410A etc. Low pressure can occur if the Copper pipe used between the indoor and outdoor unit of the air conditioner exceeds 15 meters in length.
When this occurs the efficiency of the pipes deteriorate up to 20-30% as the length of the pipes gets longer . This forces compressor to overwork resulting in improper cooling. The refrigerant consumed is also a lot more which leads to low pressure of the Refrigerant as overall volume of the refrigerant is reduced. As efficiency is reduced it may result in improper cooling of the evaporator coils thus resulting in ice.
The second reason is faulty blower motor. The blower inside the AC indoor unit is often called as barrel fan. It is responsible for throwing the air . When you adjust fan speed on the remote control of your AC its the barrel fan speed that is adjusted. As the blower motors use capacitors . The capacitors may become weak on prolonged use of the AC . Thus if the air throw is not proper then change the blower motor. The blower motor is also responsible for sucking air through the evaporator coils and then throwing through the ac vent and directing it from louvers. The images for blower motor and blower are shown below
A blower motor A Barrel Fan (Blower)
The third reason is too much dust on AC blower. It is often seen that when a blower is not serviced for long time it may lead to water drops falling from the evaporator coil of the AC. Evaporator coils are present inside the AC indoor unit.
Thus, too much dust that sticks on the AC blower motor does not allow proper suction of the air from the evaporator coils resulting in poor heat exchange and hence water droplets or ice on the evaporator coils.