1. For Canon, it needs change the AF options to Live mode by using the camera's menu button. Quick mode or Face Detect Live mode is not supported by the app. If the auto focus still does work after the above steps, please check if the AF-ON button is customized. It needs to restore to the default setting if it is customized.

1. For Nikon, it will do not work if the af area mode is face priority or subject tracking. please change the mode to wide area or normal area. how to set af area mode on your nikon 7100 (link) illustrates the steps thoroughly