First, go to my.meural.com, log in, and select the User icon in the top right hand corner. Next, select Canvases from the drop down menu. Under "Settings", scroll down to "Ambient Light Sensor" and make sure it's set to "On."
If it is set to "On", go to the Canvas, wave down, and select "Canvas info." The "Light Sensor" shows the light detected by the Canvas. If the room isn't dark, and the value is less than 15, change the "Light Sensitivity" setting on my.meural.com to "High." If the value is more than 200, change the "Light Sensitivity" to "Low."