I have been pouring Do-it molds with Lee pots for about 25 years (yes I am old). I stopped using the bottom pour spouts about 8-10 years ago (I think). I find that fiddling with leaks and having to clean the spouts is just not worth the hassle. Here are my top 4 factors for pouring that if each are addressed will eliminate 99% of pouring failures (in order of importance).
1. Temperature. About 80% of the time that the mold doesn't pour properly, it is because the lead is not hot enough. The proper temp can be different for different alloys, but usually increasing the temp by either turning up the thermostat, giving it more time or reducing the amount of lead in the pot will correct the problem.
2. Alloy. The purer the lead, the easier it will pour. When turning up the temp isn't enough, lead purity is my next troubleshoot.
3. Flow. I know bottom pour seems convenient, but I have found that a 2-3 oz ladled in the right shape, filled no more than halfway will allow you to pour the lead in the very center of the gate AND you can see what you are doing. In my experience, the flow from a ladle is superior the valve on a Lee pot. Can't speak for the more expensive brands.
4. Mold Design. I have not found an unmodified Do-it mold that I could not pour (I have over 100 of them, granted all in the 1/16 to 4 oz range).
I used to wonder why my older ones poured more easily than the new ones. I have come to believe that when molds are newer and tighter, the air doesn't escape as easily and can make the harder to pour than "seasoned" molds. One trick is to hold them a little "loose" in your hand. Don't clamp down too hard. I have an old Arrow Head mold that I had almost given up on until I learned this trick.
Now, once they are modified, they can be troublesome -- particularly using larger hooks than originally intended.
Note that I never preheat molds and have very little faith in "fluxing". This is just my take. i am sure others have different experiences.
Stephen