Thanks for reaching out to us! It doesn’t sound like you’re doing anything wrong. I’m not certain if it will give you the result you’re after, but you can keep painting over and over again and the result should become more dramatic.
But, to remove the spot of sunlight altogether using Burn will most likely not look natural. Depending on the size of the spots of sunlight you’re trying to remove, it may be better to try using Clone to copy a nearby area over the sunlight, or even some of the Touch-up tools to achieve the result. It may not be an easy task.
– Regards, The Ribbet Team