Delta 100 in Pyrocat

Hello! I am trying to get satisfying results developing delta 100 in pyrocat HD. I am new to this developer having used HC110, so I have faced a couple of problems.
The dilution is 1:1:100, 14 min at 21C (70F), 10s agitation every 2 min. Fix with TF3 1:4, 4 min, agitation every minute. I have developed 3 tank with the same solution so far. The test frame was quite good with a little too dark shadows. The second film was also OK, but the third one went out almost completely transparent, even though it was exposed correctly.
Thus the questions is what is the reason for such thin negative? Is it that pyrocat solution can only be used to develop a maximum of 2 film (unlikely, I guess)? An other reason that I see is that I have messed up with TF3 dilution. What is the optimum concentration of that?

BTW, the solution was prepared yesterday and during the night its color has changed from almost transparent to very dark coca-cola like. Is this normal?

Pyrocat-Hd is a one shot developer. I mix immediatly before using(1+1+100), then dump it down the drain and water rinse or use half strength stop before fixing.

Thank you. I knew I missed something silly and obvious.

Dilute Pyro developers oxidize fairly quickly. As Rick said, use it once and discard it. If you use Pyrocat-HD as a divided developer, it may be reused several times in one darkroom session. Divided Pyrocat-HD is used at a 1:10 dilution, much stronger than the 1:1:100 dilution used as a one-shot developer. I wouldn't keep it overnight, though.

Peter Gomena