There are several possible issues that will prevent your garage door from closing properly, and all of them are tied to the safety sensors mounted on either side of the bottom of your garage door tracks. If the sensors are not aligned so that they communicate with each other, or if the beam between the sensors is blocked, the door will not close all the way. Check to make sure the sensors are aligned and the lenses are clear of debris (spider webs can block the rays). Newer sensers have lights on them that will blink if the laser is not communicating properly. Also check the bottom of the door for leaves and other debris that could break the beam as the door lowers. Finally, if the issue arises early in the morning or late in the day, it could be caused by sunlight shining directly into one of the sensors. Try shading the sensor and then closing the door again.