mold not hot enough
I went through the same thing. I found out if I put my mold on a burner and let it get good and hot, it will produce nice looking bullets at the correct size. If my mold isn't hot, the bullets go right through the sizer, if it is hot, the bullets go through with some effort and you can see they are being sized.
I drop my bullets in cold water to keep them from getting dented up.
If the bullets come out shinny, then it is at the correct temp. If they come out dull, the lead is too hot, just turn it down a little......although they both shoot just fine. ;-)