+1
You either need bigger bullets, or a smaller resizing die, or smaller expander.
If you are sizing the .38 Colt in a 9mm sizing die, it should already have the correct expander for .355" bullets. The expander should be at least .002" smaller then bullet diameter.
But regardless of that, I don't think you need to use cream-of-wheat, oatmeal, granola, kapok, dacron, card wads, or anything else.
If the bullet has proper case neck tension, which you don't have now, the powder will burn very consistently without filler.
If you don't have proper case neck tension, no amount of crimp will help at all.
Excess taper crimp can actually make the case neck tension worse, not better.
It will squish the lead core bullet, then the harder brass case springs back a little and the soft lead doesn't.
rc