If the faces in your images are very large, then the face detector may not be able to detect the face using the default smallestFaceHeight parameter.
The face detector has a detection scale range of about 5 octaves. Ex. 40 pixels yields the detection scale range of ~40 pixels to 1280 (=40x2^5) pixels.
If you are dealing with very large images, or with dynamic input images, it is best to set the smallestFaceHeight parameter to -1. This will dynamically adjusts the face detection scale range from image-height/32 to image-height to ensure that large faces are detected in high resolution images.
Sean S
face detector fails on large images?