No difference in prior vs posterior for a parameter with uniform distribution

I am trying to build a media mix model and for that I need to have a decay parameter for adstocking the ad spend values.
Here is how I am defining the decay parameter -

As I have 17 different marketing spends and I know that all spends do not have same length of decay, I am expecting decay parameter to show that in the posterior. But I am getting almost same mu and sd for all the 17 spends(see in the image)

I need some help in understanding what are the things I should check if I am seeing seeing all the 17 decay parameters having almost same mu and sd?