Fountain Pens

As a follow up to warped, do you have the old cartridge and a blunt syringe? I can't tell if you tested with an unused or refilled cartridge. I will run with the assumption you only tested with an unused one. You can test if this is an ink compability or an evaporation issue by filling the old cartridge with the Quink. No need to fill it completely and waste ink. 1/4 should be enough to tell. Use the pen as you did with the converter and see if you have the same evaporation issues. If it's also evaporating, it is either a cap seal issue or a pen and ink combo. I second Warped's recommendation to try the ever reliable Waterman Serenity Blue (the "standard" blue shade") ink. That will give you a good idea if your pen has an evaporation issue (still drying out quickly) or a pen/ink combo issue. If there is no evaporation (no dry out and ink isn't getting darker), then you can start branching out to different inks to figure out what works with the pen. If you do not have any issue with the refilled with Quink cartridge, then I think that might suggest a converter related issue. The agitator might help. A new converter might help too. This scenario is a bit harder to diagnose to be honest.

If I was a gambler, I would bet it's either an evaporation or pen/ink combo issue.