Are they always less responsive to hard hits or only when they are lit (where a hit would light the next bumper)?

On my River Boat the dead bumpers are set the same as yours, and register every hit. They're at the top of the playfield where they don't get hit that hard.

Yours are right in the line of fire of the flippers. I'm wondering if with the stronger coils you're getting any air balls that might go over the skirts and bounce off without ever hitting them?

Maybe take the flippers out of the equation and shoot a bunch of balls by hand at them, and see if they register the hit?

Just spitballin' here...

-Paul

Maybe without the rubber rings a hard shot will send the stem beyond the spoon or somewhere else it shouldn’t go. I just learned that rubber rings go on passive bumpers as I’ve been working on a Flying Chariots. I would definitely put the rubber rings on those bumpers if they aren’t there.

They should have a rubber ring installed on them. Also, the extra power from the hot coils doesn't help. You don't have the machine high-tapped, do you?

The proximity of those bumpers to the flippers make them IMO easy meat for damage if you are using orange dot coils. I have standard coils on my WF and they work fine. Suggest returning to standard coils.

The designers didn't just guess at their work. Hot coils are a major departure from the original design, and will introduce issues such as you're seeing. Use original equipment or equivalent if you have operational issues after a modification.