THE PROBLEM: 36 frames shot but received fewer frames or even only half of frames left.
WHY: This usually occurs with disposable cameras or simple-structured cameras which it has smaller apertures. As fewer light is now passing through the lens, negatives will be underexposed and result in dark, faded, or even grainy photo. Also, the scanner might skip underexposed frames automatically as the film base is faint/see-through. As a result, you might receive fewer photos than you actually shot.
HOW TO FIX: Please turn on the flash when shooting indoor, at night, at gloomy days, and/or under shadow area.