One thing for sure your system is low on refrigerant. Need to top it up. Its difficult to tell whether there is blockage or not with low refrigerant. With that low side pressure (18 - 23 psi) when AC is ON, your system should be able to take in more refrigerant unless your refill can is nearly empty. For the refrigerant to get in the pressure in refill can should be more than 23psi (i.e more than the sytem's low side pressure). Could try immerse the can in warm water or get a full can. I'm assuming you're using the small hand-held quick refill can.

If still no any cooling even after about half-can refilling then there is blockage. No you can't knock the blockage out because it would mostly be at the orifice tube's filter mesh. Something sludge-like substance. Need to evacuate system and replace orifice tube.

In short when system charge is low there wouldn't be any cooling because not enough refrigerant to absorb heat@evaporator to release it out later@condenser. When refrigerant is enough but still has no cooling its because reduced refrigerant flow at orifice tube due to blockage.

Amarin.