Could the sponge foam/filler be disintegrating?
I found a carwash sponge in my garage a few weeks back that hadn't seen the light of day for nigh on 20 years. It crumbled in my hand.
Proof that my concern about sponges dying is valid.
I have a friend who spent a working lifetime in plastic manufacture who once told me that his company, wherever practical, put additives in them to ensure they bio-degrade.