Alignment is Critical
The main garage door sensor that most people have trouble with is the sensor that monitors the threshold. These are typically light-based sensors -- older ones used a simple light bulb, newer ones use infrared beams. With a smaller, single-width garage, this alignment is not so difficult. However, larger, double-sized doors require more precision. It your sensor is not working properly it could be merely out of alignment. Loosen the mounting bolts on the garage door sensor and check the aim; often, an LED will light on the receiver side to show that signal is being received. An extra set of eyes can be very useful here if working in bright sunlight.
Another helpful tool is a long piece of string that can be stretched from garage door sensor to sensor. This can help with height adjustments. Once realigned and working properly, tighten the mounting bolts. Loose mounting bolts on garage door sensors are the result of normal vibrations during use. It's a good ideal to check these often.