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I did go stright to the mags after shooting about 100 rounds of .38
This is the reason. The .38s are shorter than the .357s so when you're shooting .38s, there's about 1/8" of freebore space in the cylinder that gets lined with a ring of crud. When you then shoot full length .357s, the case expands into the crud ring, making extraction difficult.
I bet your problem would go away if you fired the same ammo, but went with the .357 first and then switched to .38.