This is a difficult question to answer, because postmodernism is directly in contrast to all theories. Postmodernism is a believe that all knowledge is relative and that there is no such thing as a meta-narrative that subsumes all knowledge. To argue that there is a meta-narrative, according to postmodernism is an exercise of power and control.
With this stated, postmodernism rightfully goes against theories like positivism and modernism, which believe that there is an objective reality that one can reach through reason and rationale. However, there are more nuanced social theories that postmodernism needs to wrestle with. In particular, postmodernism needs to wrestle with scholars who study the sociology of knowledge.
Scholars such as Thomas Luckmann and Peter Berger have successfully written how knowledge is socially conditioned, which accords well with postmodernism, but these scholars have not done aways with the pursuit of non-relativistic knowledge. For example, Peter Berger wrote a book called, The Rumor of Angels, where he argues for some objective reality, which he calls signals of transcendence. Postmodernism, in my opinion, falsely and wrongly rejects scholars like this.