Bacillius Subtillis

[Aria McCall]

I am using Bacillius Subtillis in the AOAC antibiotic plate method for 
Lasalocid.  Lately I have run into a problem where the lower 
concentrations of the standard curve 0.3, 0.36, 0.420 micrograms/ml have 
not been producing zones.  The higher concentrations have smaller ragged 
zones.  We have tried diluting the concentration of the inoculum and have 
even made new inoculum from a new spore.  None of these have solved the 
problem.  
Any suggestions?

Aria