According to my observation you just centrifuged the cultures and used supernatant to access the antimicrobial effects. Centrifugation is usually not sufficient to separate cells so when supertanats of centrifuged cultures are used the results like this appears. The easiest method would be to filter supernatant with 0.2um filter before application on agar. It will work and give you better results. Good Luck

i also use filter the situation is same 

which pore size filter?

Может срабатывает киллер-фактор бактерии? Или я вас не так поняла ?

yes it kill bacteria