If the system does not have enough refrigerant in it, ie the pressure is to low then the compressor will not go on. You may first need to add refrigerant with the compressor and vehicle off and then once the system is pressurized somewhat you can then add more, with the compressor on in order to get the high and low pressure correct based on the ambient temp. I don't know how much oil came out when you installed the pressure switch but you may need to add a very small amount of oil, to replace what you lost.