My thoughts are that maybe the rubber part has got coated with all the grunge of the scrubbing.
And that maybe if it were scrubbed up a bit with something rough ...
...like maybe one of those green kitchen nylon scrubbies to help get it back to life?
Good Luck!
However ... maybe this is a question to ask the manufacturer?
If it's happened to you .... it has probably happened to others before,
so they should have answers for you.