I too had a problem with incomplete castings at first, especially on smaller jigs, so I started preheating my mold by setting it on top of the melting pot for a couple minutes, and that has taken care of it. I'm a total newbie to this, and maybe there's a good reason for "smoking" molds, but I think it's important to note that, on D-It molds anyway, doing so voids the warranty. Here's the web site:
http://www.do-itmolds.com/tips_limited_warranty.aspx

and it says:

Warranty is void:
if mold was altered, disassembled or otherwise modified.
if mold was preheated over an open flame.
if mold was forced open when lead was poured into the hinged area.
if mold was abused by being dropped or handled in a reckless manner.