Some beans, especially the "roasted so dark it's nearly charcoal" variety can be way too oily. The characteristic is that rather than grind to a powder it turns to "paste" in the lower chamber. This is obviously bad, because it then comes out in clumps or not at all.
Clumpy grounds, for some reason tend to overshoot the portafilter and make a mess. I suspect this is because the large clump size has more mass, and comes out of the chute with too much momentum.
If you pull the upper burr, and see what looks like a nice smooth black surface where the impeller would normally be, then that's your sign.
The Fix: Clean the lower chamber, or at least break up the cake of grounds, and switch to a less oily roast.
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