demo.py no masks on custom dataset, works on coco.

[Matimath]

I have run demo.py on coco dataset and it works.
Then I tried to train (overfit) the model on 4 pictures from other dataset. During training all the accuracies were strictly positive (lowest were around 0.5). Nevertheless when I tried to run demo.py on them I get only the original image.
Do you have any ideas how can it happen (in particular positive accuracies during training and then no results for demo.py).