In addition to an overwhelming task of trying to capture all words and expressions that can convey a sentiment there are many other problems to solve: resolving the scope of negation to determine the shift of polarity (Lapponi et al., 2012), determining if an opinion is present in interrogative or conditional sentences (Narayanan et al., 2009), dealing with irony (Tsur, 2010), etc.
the recall for the positive class is substantially low than for the negative class.