I think the older ones clump when the coffee comes out so fast it piles up behind the metal finger guard. Some coffee clumps easier than others too - especially dark or oily coffee. As fast as the coffee comes out the clumps should shatter against the chute before falling into the basket. I taped my finger guard flap in the open or "up" position with good results.

This is a raw photo - no pre-cleaning and for the past week I have been working with Deep Cello's Nostromo espresso blend which is a delicious dark roasted bean. The finger guard looks sandblasted clean.