Hmmm, I take my Sims adventuring a lot (I love WA) and I must say I have never came across this. First off, some questions:
1: What tomb is it? My Sims just went to China recently, chances are they explored the same tomb, if
there is something you are missing I might know. (No promises).
2: How old are your Sims? It's possible that teens and kids can't do this, I've only ever taken young
adult Sims traveling so I don't know what other age groups can do while exploring.
3: Do they have the fear moodlet? They will refuse to touch traps if they have that, also if their
mood and needs are too low they won't do anything either.
4: Do you have any custom content installed? This can cause bugs.
Make sure there is nothing in the way between your Sims and the trap, put wallls up and mouse around, maybe they just can't get to it. That's all I can think of for now, I've heard of people getting bugs on vacations, haven't heard of this one although that is what it could be a bug, it sounds like it. If it's just happening in one tomb then maybe try another? Can you go back to a previous save file and try again? It might be a corrupted game file so try that. I don't really know, I'm just trying to think of things, sorry if it seems a bit useless.