Some rough calculations:
Assuming the flash and the subject are located at 5 ft down from the ceiling, if the light is bounced off the ceiling, it has to travel 14 ft from the flash to the subject.
Assuming the flash head is at 28mm, the guide number then is 102 ft. The f-stop before adjustment for reflectivity of the ceiling is 102 / 14 = 7.3.
Bouncing off a popcorn ceiling can lose as much as 3x of light (only 1/4 of the light from the flash reaches the subject). In this case, the (rounded off) f-stop has to be: 7.3 / 2 = 3.5.
This explains the underexposure at ISO 100, f/5.6.
Most flashes (even your car's headlights), when old, become less efficient. Thus the guide number with the head at 28mm can be less than 102 ft.
A flash lightmeter is very helpful to debug exposure issues.
Last edited by SOldBear; 02-09-2016 at 07:51 PM.