Hmmm, they seemed to work when I tried them, but I did so only once.
Once you place the detonator, it should be visible on the tile. Check whether it's there. If not, then you did something wrong when trying to place it (clicking the wrong button or sth. similar).
If it's there, it should explode when an enemy crosses the tile. I don't know if it will explode when friendly units are in the explosion range - this might be a problem since there's code in the epic game which checks for friendly units before you can e.g. drop a nuke. But that's a very wild guess.
Due to the limited visibility range, I think it might also happen that a detonator gets triggered without you seeing it.
Also, I have only tested one detonator, there might be bugs when using many at the same time, or they might be bugged like the sentinels which can only be used once.
A save game might be helpful.