This can happen if your model is too large.
Here are some recommendations to make it smaller:
Add up to 50 documents per collections
Add 2-3 samples per layout type (It is not necessary to train similar layouts from different vendors, same terms, same kind of field positioning)
Training process
1. Train an model adding 10 collections
2. Test various vendors with this model
3. Restart at 1 with vendors not properly captured in 2