To be fair:
Saturn - Failed to kill Robin. And also he deadass caused this entire mess by killing Vegapunk
Killing Vegapunk was the objective, they had no way of knowing this recorded speech could've bene a consequence.
Vegapunk was simply one step ahead of them.
Mars - Unable to find a snail.
It definitely isn't as easy as it looks, as said in the recent chapters the Transponders are all sentient beings, and in an Island like Egghead (literally the most futuristic out there) it's fair to assume that Transponders are all over the Island. Just think of the security cameras...
Not an easy task to accomplish in 10 minutes, the only solution would be to destroy all the sentient beings on the Island, a conclusion the Elders have already reached at this point.
Also, I think in the end the Main Transponder will be revealed to be
inside York. Which would be a real slap on the Elders face since she was their very own spy to begin with.