Hi Vladim
It’s because the subject lacks “features” on the surface. That means flat featureless subjects are always a problem to scan using photogrammetry. You need to use pattern projections or some powders to get it on a model…
I recommend you to take a look at James Busby’s nice tutorial regarding scanning transparent stuff, which is almost the same as featureless subject in your case: http://www.3dscanstore.com/index.php?ro … post_id=19