Could you try printing out the set of classes that are predicted by your network? That is, out of the 10 (or 11, if you count “nan” as a valid class) classes in your input, see how many the net reports as having seen. It is possible that your network is predicting only one class, or a couple of classes.
Also: I seem to remember that the F1-score applies only to binary classification? But your input has more than two classes?