One possibility is that "old" brass had deeper primer pockets. If so, the primers may not be fully seated in the casehead and that's an invitation for misfires. Another possibility is that your primers were exposed to some moisture in storage and this has resulted in primers that aren't quite as sensitive as they need to be in you model 19.

Another possibility that comes to mind is that your firing pin needs to be replaced. Note, this is assuming that your running your model 19 with the factory mainspring setup and the strain screw hasn't worked loose. With the hammer mounted firing pins, checking this is very simple. Open the cylinder, pull the cylinder release to the rear and drop the hammer fully forward. Now take a dime and lay it alongside the tip of the firing pin projecting through the recoil shield, if it's the correct length it should be even with the surface of the dime.

BTW, this test doesn't work with the frame mounted firing pins because they only project through the recoil shield by about half as much. Note, I'm still scratching my head how firing pins with a much shorter projection can prove reliable but I haven't had any problems with misfires on my newer guns with the frame mounted pins. I also tried an extended firing pin in my 610-3 and put the standard pin back in when I saw evidence of microscopic leakage in about 30% of the rounds fired with the extended pin.

Final possiblity is that the tip of the strain screw in your model 19 has mushroomed enough that it's reduced the power of your hammer strikes, or the mainspring needs to be reshaped. I've heard, but not tried it, that placing the mainspring on a table with the arch up and tapping it with a plastic hammer will "reset" the mainspring so that it will again function as new. It might be worth a try doing this, however I would trace the current arch on a piece of paper and do a bit of comparing while doing this. Take about 1/8 inch of the arch out at the midpoint and it may solve your problem.