Heat treatment along with phenol treatment can work.as it might be possible that bacterial colonies suppress the growth of actinomycetes.actinomycetes are known to be grow from day 3 to day 28 depending upon the medium and culture condition as well as the climatic condition of the sampling area.You can use the AGA as well as humic acid agar or ISP medium.Try to suppress the rowth of bacteria during isolation and use proper antifungal agent.