deadly conditions.

[Suicide Junkie]

Note that the conditions improvement plants only kick in on every 10th turn. (turns ending in 24XX.0)

Given 300 turns though, and a CI 3 plant, the planet would have improved by +90%

Were you running a finite resources game, perhaps?
For finite resources the value improvement plants multiply with the value, rather than add. Conditions improvement may do the same thing, and as we all know, zero times anything leaves you with zero

PS:
They do stack fully, so building 5 CI 3 plants on a planet makes conditions improve by 15% per year.