Culture from milk & milk products

We are isolating Listeria from milk & milk products by molecular biologically and & conventional culture method. We are targeting to hly A gene using LM1 & LM2 for 1st PCR and LL5 & LL6 for nested PCR. We have got most samples are PCR positive but we are worrying about some samples we could not isolate from conventional culture method which are molecular biologically positive. For culture we used selective Oxoid agar media. These cultures appear black haloe similar to Listeria but doing gram stain and other tests Stapilococcus sp, Streptococcus sp , Micrococcus sp isolated. Please suggest a solution for this problem