On my big dcxl which happened to only have about 3 inches of depth on the soil, I noticed that the bottom 2 old traps which no longer have heads turning black. I am guessing that this may be a drainage problem so I added another 4 inches of soil in order to give it better drainage.
Could this simply be older traps sacrificing energy for newer traps? This is the first time I've seen it happen. Those 2 old traps have also stayed closed for 3-4 weeks already so I assume they will never open back up.