Pyrocat HD maintains its strength for extended periods of time. Six months is a reasonable period if stored properly to expect it to perform as intended. Contamination is a serious problem that I have experienced first hand. I got some solution A into my solution B and it turned the B solution a tawny brown. I had to discard my B solution. As a result when mixing developer volumes, I use separate pipets and suction bulbs for each solution to take the contamination variable out of the equation.
One of the variables that I believe many are taking lightly as evidenced by reports of thin negatives with Pyrocat HD is carefully measuring your developer temprature with a process or an accurate digital thermometer and adjusting your development time accordingly. If your developer tempratures are lower than expected and you develop for the time recommended for the higher temprature, you are taking a double adverse hit on the results. Just my $0.02.