Most modern self-closing hinges have an adjustable mechanism inside that you simply turn to increase the tension and closing power. If you push down on the mechanism, it releases all tension. Use the appropriate sized allen wrench and turn the mechanism to increase the tension, which will close the door. It may be that the single hinge is not enough to close the door, and you may have to install a second hinge on the door. If so, install the hinges at the top and bottom hinge positions on the door.
You want to adjust the hinges so that the door doesn’t slam closed, but that there is enough force to close and latch the door. This may take a little fine-tuning of the hinges so that you can accomplish this. The final test is this: hold the door at a 45-degree angle and let it go. It should close and latch. If it doesn’t, you can gradually adjust the hinges until it works.