What's cooking Kaggle - Improve model
But the problem of this dataset is that we have unbalanced data


I think that the way to fix your problem is to use something like SMOTE or one of its variants. (My favorite is SMOTE-ENN, but go with what works the best). Here is an implementation that you can use to fix the class imbalances. I don't know if this will solve your specific problem, but it is worth a shot.