Trying to create a model on a larger dataset - Loss stuck at the same number and not moving, resulting model predictions detect nothing

[az-oolloow]

I tried turicreate on a small-ish dataset (a few hundred images and some 20 classes) and was quite happy with the result.
I gathered my full training set to get some 7200 images, 273 classes and some tens of thousands of annotations.
When I feed it into turicreate the model always shows me 0.0311279 loss and it is never moving. No matter how many iterations I go through, resulting model doesn't detect anything when trying to get predictions.

Is there any way to understand what is going on?