The system is running but the air is not very cold, what's wrong ?
If you have some cooling, but not enough, then chances are that the system is low on gas. The only reason a system would need gas is because the gas has leaked out. If the leak is not sealed before recharging the gas will eventually leak out again.
Leak repairs are expensive so if it's a small leak it may be more cost effective to recharge the system every two or three years. Gas recharging is not a do-it-yourself job. Only people certified to handle refrigerant gas are allowed to recharge an air conditioning system.