What to do when facial recognition fails to find a face?
I guess you have a learning problem that has low number of training data. I recommend you a two-step solution.


First you need to do error analysis. Find the images that your classifier does not recognize and try to compare with those which your classifier correctly classified them. find the features that your classifier can not do well on them and try to add features that your classifier can discriminate those, I mean try to find yourself what prompts your classifier to make mistake on those images and why you don't make mistake during recognizing them. Try to add those features that you care but your classifier does not and add them as input. 
Try to augment your data using data augmentation for both you mistake and those which you don't. There is an important fact here. don't change the distribution of your data. I mean if one class has two times bigger population than the other class, after augmentation try to keep this ratio. The reason is that maybe after equalizing the number of population of classes you reduce the Bayes error but at borders of classes the answers would be random.