Hi,
I had the same problem. I found the the lee pot(i got the 420) does not get the lead hot enough when filled up with lead it will perform a bit better with around 3" in it.Then it can get hot enough. I basically use my pot as a pre heater for the lead. I ladle pour everything now heating it with a propane torch. hotter lead pours so much nicer,its bright, clean and shows great detail on molds with scales and lines. the other possible problem is you are not filling the mold fast enough, adjust the flow for a faster pour the lead is cooling a bit before it is filling the collars. I almost positive heat is the problem.... get a ladle and give it a try (Hotter is better) it may save you some frustration and save you some time remelting the bad ones.
hope this helps...
Chaotic baits