This can happen from time to time, either offline or "live" mode. I have seen where the cellular signal is marginal and this will cause the point submittal to time out and fail and you get no saved point or photo. Also, I have seen where the point submittal with photo shows as successful and later, no photo is attached, though this is pretty rare.
We found the safest process to take the picture on the mobile device via the camera app, then while in Field Maps or Collector, select your photo from the mobile device photo library prior to submitting the collected point. That way you always have a second copy of the photo should it be needed. Taking the photo from inside FM or Collector does not store the image on the mobile device, it only transmits it.
It takes longer, but you can also verify successful point submittal on the mobile device by selecting the point from the device screen and reviewing the attributes and the photo prior to moving to your next location. This helps with a variety of field data problems that come up.
One other way that points/photos can be 'lost' is when the field user incorrectly saves them to the wrong editable layer, i.e., he has the wrong map open on the mobile device, in which case they are not really lost, they are stored in a different map service.
Hope that helps some.
