First of all, don't use "Trigger once" when multiple instances are involved, this is a common mistake which always causes problems.
If you have multiple pressure plates and doors on the level, you need to use "For each plate" as the top condition. Also, you need to add a "state" variable to the plate. Here is how I would do this:
For each Plate
Enemies overlapping Plate
or Box overlapping Plate
Plate state="up"
: Plate set state "down"
: Plate move to (down)
Else
Plate state="down"
: Plate set state "up"
: Plate move to (up)
You can also add all objects that can step on the pressure plate into a family, this will make things even easier.