Poison, locust or think like that are set so that they stop at 1 HP.
Try triggering it so it locate a unit with less then or equal to 1 HP and instantly kill the unit (as condition, the unit must have poison effect or locust effect)
I just tried it on a building with 5 hp, and -1 health regen rate. It worked fine for me. Try placing a unit on the map, and watching to see if it dies.
Do you have any triggers that have to deal with preventing death, or when a unit takes damage?
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.