Does the door just stop or does it stop and reverse?? If it just stops, flip the cover down and you will see two black knobs. These are your force adjustments. Turn the down force up slightly until it is consistantly closing. If it is reversing, look at the photo eyes. One will be green (sending eye) and the other will be red (recieving) the red light will be blinking if they are out of alignment. If this is the case play with them until the red light glows solid.

Two things with genies, If you play with the sensors and light does not glow a solid red, the sensors are bad. Very common problem with genie.

Number two they are sensitive to sunlight, more so than other openers. SO is the problem occuring all the time or at a certain time of day.

Hope this helps