The switch or the motor may be bad. When it happens, try wiggling the switch a bit when you are trying to open the window. When switches start to fail, you can often jiggle them a bit and get them to work briefly. If the motor is failing, try this. When it's not working, open the door, try the switch and slam the door with the switch depressed. Just like the Fonz used to smack the jukebox to make it play, you need to give the motor and the switch a little "encouragement" !