How can DNA results be positive and then negative?

Sloppy lab work. That's the only way, short of a crime like bribery or conspiracy. Mistakes happen, contamination, sample degradation, poor technique, machine error - and if the lab won't explain, in terms of lab error, how they could have given that much confidence and now have none, they are disingenuous to say the least, and probably owe you a refund. I mean, how could you believe anything they say, if they certify two completely different results for the same DNAs? What good are they for anything? DNA testing done correctly gives a *repeatable* very high confidence result. At the least, get hold of the guy who bragged about his experience and put him on the grill.