I think the Imps might get bugged with large dungeons, or watery dungeons, or otherwise unknown conditions. As you can see from this Screenshot (see the idle minions report), they idle around traps they should be building, even with the Priority Flag set on the traps.
I have an 8-prop Foundry stuffed with parts, so I know this is not the problem.
The only way to make those Imps work is to drop them inside the Foundry, where they will pick up the parts and bring them to the traps.
However, as soon as they have brought the part they were carrying, they will go idle again, even if the traps are unfinished.
I've tried many things (slapping, summoning new Imps) but it all failed thus far.
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