A garage door that fails to close completely can pose a real problem. There are a few common causes of this issue, and they are all fortunately repairable. 

Faulty Limit Setting

The limit setting determines how far the door needs to close before the motor disengages. This prevents the motor from continuing to run after the door has touched the floor. If the door is stopping short of closing by only a couple of inches, without ever fully touching the ground, then a repair technician may need to recalibrate the limit setting to the appropriate place for your door. 

Obstructed Sensors

The safety "eye" sensors are located near ground level on either side of the garage door. Over time, they can become knocked out of alignment or become covered in dirt. This means they will sense an obstruction and stop the door from closing even if no obstruction is present. Wiping off the sensors and then making sure they are aligned properly should help solve the problem.

Poor Track Alignment

The garage door track can come out of alignment over time, either due to warping or from items banging into it. When this happens, friction may prevent the door from rolling all the way closed. Fortunately, the tracks can be re-aligned by adjusting the alignment screws that hold them in place. If a track has become warped, it may be necessary to have it replaced. In some cases, cleaning the track can help as a dirty track may lead to similar friction issues, such as poor alignment. 

Broken Cable or Springs

Your garage door likely has a spring on either side of it, and then a spring or cable in the center. These cables and springs maintain tension and aid the opener in raising and lowering the door. If they fail, the door may only close partway or fail to close completely. You can usually spot a broken cable or spring with a quick visual inspection. Leave the repair to a professional, though, as the high tension of these components can lead to injury if not handled properly.

Disconnect Enabled

Every automatic garage door opener has a disconnect cord. If the cord is pulled, the door is disconnected from the automatic opener so that it can be operated manually in the event of a power outage. If you can easily open the close the door manually, but the automatic opener won't operate, then chances are the disconnect cord simply needs to be re-engaged. 

Contact a garage door repair service if you need more help with a door that fails to close.