I do 4oz. spros, and 6 and 8oz. fluke balls all the time. Are you using good lead, or wheel weights?
Is you spout opened up? Meaning that it's not clogged with dirt and the flow is restricted. When doing the larger stuff, I make sure the pot is full so there is more weight to push the lead out of the spout faster.
Sounds like you are heating the mold, but after you get the mold hot enough, leave the hook sit in the mold for 15-20 seconds so that heats up also. In the colder months, I have to do this.
What type of bottom feed mold are you using? A Lee pot? Worst case, you may have to drill that spout a little bigger. I know others that have an issue with the Lee pots and larger jigs.
I have had your issues before, and warming the hook, and using cleaner lead helped. You may also want to try that mold release spray. I don't use it, but others here swear by it.