sensors are likely mis aligned. both green leds should be lit steady.

On my brother's, the sensors were misaligned or broken.
check the lights on your sensors to see if they are flashing, if so, they are probably not lined up correctly. The LED may not be green though, my brothers is red.

I recently had this problem. Turns out that the sensors WERE lined up properly, but the wiring came loose up on the opener when I accidently disturbed while changing the light bulb.