How can I improve the recall of a certain class in a multiclass-classification result I am working on a multiclass classification which is to assign medical related queries of web search to certain departments of hospital.My classifier is based on the fastText. 

I found for most conditions, the result is good enough say recall is 0.8 for Nephrology. However, for just one department, Dermatology, the recall is pretty low,like 0.5. Unfortunately, this label has most samples in the test data.

How can I improve the recall of one class while maintain the performance of other classes? Will ensembling method work?
Recall is the ability of a search model to find the correctly labeled items amongst all the items for a given query.

One common method to improve specific query results is to create a custom model. Create a model just for "Dermatology". That model can be tuned to increase recall without impacting other queries that can use the generic search model.