How to make smooth camera postprocessing rendering featres slidertransition

i want to creat my own storyboard post process material like the one from this learning cours below

I created the same post process material


and add it to my camera setting
and picture(texture) apears fine but it does not blend smoothly.
it just apear with no opacity


I’m using ue5 but i think it’s not about ue5
the ue5 version of exaple project does not have that kind of issue

anyone who knowes the solution?
thank for read