If there is no suction into the return vent you should make sure the return duct is attached to the unit.
Also check the direction of the blower wheel on the inside fan motor.
A bad fan motor capacitor can cause the indoor fan motor to run in the wrong direction.
A dirty evaporator coil can cause this problem.
Loose insulation on the evaporator coil housing can block the air flow.
Low refrigerant causing the evaporator coil to freeze over can cause this problem.