Central AC not cooling house?

Were renting an older house (renovated) with two floors, an underground stone basement, and a walled-in wrap-around deck. All rooms in the house have working vents for central heating and air conditioning. Now that its getting warmer our we want the house cool but were having a problem.. its not really cooling. The temp seems to be stuck at 75 although we turned it down to 50. Cool is on and we've switched between the fan on the system being on auto and on. Very cold air is blowing out of the vents but the house is still warm. We have ceiling fans in almost every room on and tonight I tried closing ALL doors to all rooms, and all lights are off. The front door that goes to the enclosed deck (which has a door to ultimately go outside) has a glass pane that is broken , so it may let air out/in .. but the deck is wall-enclosed with glass Windows so I don't see that being a problem.. plus right now it feels colder outside than in here! There are 6 people living in the house but its a big house and we've been sure not to block vents. Anyone know what's going on?