As per the writer's opinion, the camel is unwilling to work as being a lazy animal. After he got the hump, the Djinn asked him to go out of the desert to help the other three animals. It appeared that the camel still did not pay heed to the Djinn and worked lazily in the desert itself. It is for this reason that he was still unable to complete the pending work of those three missed days; and he continued to  carry the hump and living in the desert.