Based on your light condition, the threshold predefined for on light bright might not be good enough. In such case, we can use set light threshold to tune this value. You can make it a trial and error approach (using a binary search) or use serial to log values and do a deeper analysis (We'll dig deeper into logging and analysing values in another guide). In this case, we've just approximated the best value for the particular setting.
We also beef up the brightness of the LEDs to the maximum (255) to get the brightess flash possible.