I guess I understood.
for these new classes, are the number of training instances similar to the number of the original classes?
another thing you could do is change the model spec to one with higher accuracy (I think you are using EfficientDet 0, maybe try the 3 just to see the accuracy)
Changing the model spec might help a little but my gut feeling is that there’s something in your dataset.
Another thing you could do to have more information is to plot a confusion matrix and try to understand where your models getting lost (which class it’s misclassifing and need more samples)