vowpal wabbit : multilable_oaa does not return label for all inputs

Question

I have to predict two labels and I have created a model using :

model = vw("--multilabel_oaa 50 --ngram E2 -b 24 -l2")

This returns two labels in 65% of predictions. But in the remaining cases it returns only one label or even three labels in some cases and also empty predictions.

[12, 44],
 [12, 47],
 [32, 44],
 [25, 44],
 [27, 28, 44],
 [44],
 [25, 44],
 [11, 44],
 []

I have tried with two separate models using oaa to predict the labels. But wanted to compare it with this approach but I am unable to get the expected predictions.