media for increase antibacterial compound production from aspergillus niger

hi
i have an aspergillus niger isolate have the ability to produce antibacterial compound but when i left the isolate for 6 month in the refrigerator after that when i check it again i found that it lost its inhibitory i wonder about the resean and at the same time if there is any media can i use to enhance its production ability .
thank you
rsa