I have the same problem when loading 28ga #8's in my PW-375. I've tried several different things but found that pulling the drop tube up slightly off the wad before dropping the shot & then tapping the tube lightly with a small brass hammer helps prevent the issue 90% of the time. The other thing that helps on the 375 is to drop the shot slowly rather than just throwing the drop plate over to the powder side quickly like I do on other gauges.
Two of my buddies have the same problem with #8's hanging up when loading 28ga on their MEC loaders as well. IMO, what it comes down to is the I.d. of the drop tubes is main culprit for this when loading the smaller gauges and it's just something you have to deal with, it's frustrating for sure and I can only imagine what this might be like when loading .410's.
The other thing this points out vividly for me is why you don't want to shoot steel shot in tight chokes. When I experience #8 lead shot bridging on my loading press, I can imagine what large size steel shot could do going through a tight choke.
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