Parking in Lisbon

Please explain its a mystery still!!!!

I put money into the parking meter and nothing comes out again not even the money...I know its a new strategy by the government to collect money but heyI need to avoid this and also parking fines :)

For instance what is the point of those ticket machines next to the road near Cais Sodre where you park on the road you put money in and no ticket ever seems to come out...and often I am too tired to read what the machine says in Portuguese.

Probably I am being a bit not getting it but would appreciate a simple explanation??? For now I just park there and normally dont pay anything but...