I've recently been performing many pcr reactions and have only recently run into contradicting results. What once was coming out as positive is now negative. I run positive and negative controls. I use the same materials everytime. Only thing different is a new reaction to check for reproducibility and show expression over course of development. I use GAPDH ase my positive control which always shows up unless in my negative control (no RT) but my intended band appears sometimes and sometimes not... What's up with that?