Welcome to the new forum, Tim.
So, with conflicting testimony, who is to be believed? We do know that the FBI laboratory ran multiple tests and all yielded a positive reaction on the casts. However, Cunningham never stated how many tests they ran before casts yielded a negative reaction. He only provides the results of a "Mr Killian". Of course the question is never asked "how many times", and thus we are led to believe that there was just one test. If there was only one test and the results were negative, it was clearly a fluke (considering the multiple positive tests). If there were multiple tests with one negative result, how many? We then could play the odds to determine whether or not it was likely that LHOs tests would have a positive reaction.