It could be that only one message is marked as 'negative'. Therefore, the system is fed with only one (negative) sentiment.
Our advice is to mark all your messages with sentiments (positive/negative/neutral). By adjusting the sentiment of a message manually, you can learn the system to assign a sentiment automatically. After having manually labeled about 40 to 60 messages, they will be automatically marked with a certain sentiment. This will improve the probability of the sentiment.