A simple remedy is to use word bi-grams in addition to unigrams (Blitzer et al., 2007;Glorot et al., 2011;Wang and Manning, 2012).
use of word n-grams with n > 1 on text categorization in general is not always effective; e.g., on topic categorization, simply adding phrases or n-grams is not effective (see, e.g., references in (Tan et al., 2002)).