Usually after child birth wife gives more focus to child and unknowingly husband is ignored. In many cases after child birth you put on lots of weight, your beauty lost its appeal, your body changes shape, this can affect relationship in many cases. Another important reason could stress
at workplace or in family. Try to talk to him, make him please by good food, start taking interest in his work, and give him time. Maintain your self well and when everything goes well then only talk about 2nd child.