Segmentation with imbalance dataset

Hi Everyone
I Am training Unet for binary segmentation with an input size of 2048x2048 and due to an imbalance class in my dataset am using class weight. I Am training with a combined (BCE and Dice) loss. when evaluated on the test set, the model does well on the positive but also predicts the negative class positive. what are some suggestions I can to resolve this challenge? any help is greatly appreciated.