phage titer problem - (Sep/23/2008 )

i am having problems reproducing a phage titer using the agar overlay method. I am working with Bacillus subtilis and a lysogenic phage SPO2 (mitomycin C is used as the inducing agent). Initial attempts to titer the phage were successful but subsequent attempts have not been. Instead of individual plaques, i only see clearings of no growth. I have tried to increase the dilution up to 10^13 but still, no plaques. I've also tried with new Bacillus subtilis cells. I am at a loss of what to do next. Any advice would be greatly appreciated!!!