Check the Photo-Eye Sensors First
The single most common reason a garage door won't close is a pair of small safety sensors mounted a few inches off the floor on either side of the track. If they're misaligned, dirty, or something is blocking the beam between them, the opener will refuse to close the door — or start closing and then reverse back up. Look for a blinking or solid light on one or both sensors, and check the opener itself: most units flash an error light or code on the wall control when the sensors are the problem. Spiderwebs, dust, sunlight glare, a stray leaf, or a sensor that's been bumped out of alignment can all trigger this.
Other Mechanical Causes
If the sensors check out and the door still won't close, the cause is usually something mechanical. An obstruction sitting in the track, a track that's been bent or knocked out of alignment, or a limit-switch/travel setting on the opener that's drifted off can all keep a door from closing properly. More seriously, a frayed or broken cable, or a spring that's worn out or snapped, can make it unsafe for the door to close at all — the opener may sense the resistance and stop, or the door may hang unevenly. Any of these can show up gradually, so a door that's been closing a little rough for weeks is often headed toward one of these failures.
What to Check Yourself vs. What Needs a Pro
Safe DIY checks: wipe down both sensor lenses with a soft cloth, confirm they're pointed directly at each other with steady (not blinking) lights, and clear any debris, tools, or storage items from the track area. That solves a good share of no-close calls on its own. What's not a DIY job: anything involving the springs, cables, or the opener's internal limit and force settings. These parts are under heavy tension or control the door's travel, and adjusting them without the right training can cause real injury or damage. If the sensors are clean and aligned and the door still won't close, that's your sign to call a professional rather than keep testing it.