The trigger bar spring must lift the rear of the trigger bar and keep it up until the cartridge is fired and the blowback gasses move the slide rearward where the ramps under the slide knock the trigger bar down and disengage it from the sear. In DA the sear is never used to catch or release the hammer.....however, it must be kept rotated out of the way of the falling hammer by the rear legs of the trigger bar.
When this no fire, no firing pin issue arises....take a careful look at the position of the hammer. Did it fall all the way down or did it get caught by the secondary hammer hook? That is what happens when the trigger spring gets too weak. It doesn't keep the sear rotated out of the way of the falling hammer and the sear catches the hammer before it reaches the firing pin.
A quick and easy test to see if this is the issue is to take a short piece of tooth pick, stick it behind the trigger and under the lower trigger spring legs. This will retension the trigger spring. If that cures the issue then you know exactly what the problem is....an overwound and too weak trigger spring. It is easy to unwind and restore proper tension but you must disassemble the entire pistol including the frame halves. If you don't enjoy this type of stuff....let Walther fix it.
In SA, the sear is holding the hammer cocked. Pulling the trigger raises the rear of the trigger bar, the rear legs engage the sear and rotate it off the primary hook and holds the sear out of the way of the secondary (half cock) hook as the hammer falls all the way forward.
In DA, the trigger bar rises, first engaging the hammer strut. Continued pull of the trigger rotates the hammer rearward via the hammer strut. As the trigger is pulled further rearward, the rear shoulders of the trigger bar engage two slanting pins molded into the side of the frame. These begin pressing the trigger bar down until it allows the hammer strut to disengage allowing the hammer to fall. The sear never engages the hammer hooks. However, the trigger bar must rotate and keep the sear rotated out of the way of the falling hammer even though the sear is not engaging the hammer. 1917