How to overcome the class imbalance issue in a text classification problem?
Option 1: play with the thresholds for classifying something – perhaps you could set a threshold for classifying something as a minority class should even if it doesn't have the highest probability/score, or a ratio of probabilities/thresholds.

Option 2: upsample your data (take resamples with replacement of the minority classes until they have as many observations as the others).

Option 3: use some cost-sensitive algorithm (e.g. some forms of classification trees) - probably won't translate into very accurate results.