Edge Effects

Where was the plate incubated?
Often times the conditions for ambient incubations can be less than ideal. If there is a draft in the area or the plate is incubated on a cold lab bench, this can lead to uneven color development.

If multiple plates were run, were they stacked on top of each other during incubation?
Multiple plates should only be incubated in a single layer. This will assure that no area of the plate is at a different temperature than any other.