Answer

Possible cause:
The disc packets have been opened for more than 1-2 weeks before being used, as they have quite a short shelf-life once opened. They should be stored unopened in a fridge.
The  bacterial cultures were not viable, or the incubation period was not long enough (or the temperature was too low/high).
 r, of course, the bacteria is a type which is not killed by penicillin.