I have had this same problem at our 4 family. Before ANY tow company will tow, it has to be ticketed!
Police do not want to ticket on private property. They do not want to start a precedent -- as they say they don't have enough manpower. Also, I think they don't want the liability involved -- if they ticket the wrong car, then they have to have police in court, etc.
However, the police in the town I LIVE in, say they will ticket on private property, but it has to be properly posted... they will ticket for parking improperly where posted or something like that.
The sign has to reference the proper wisconsin statute... I believe the statute is 346.55(3) Drive around and look at signage posted at the apartment complexes, stores, etc. in your area. Double check with your police department regarding their ticketing ordinances.
Good luck and let us know what happens.