Like many budget products , Lee is built to meet a certain price point....sometimes you get what you pay for .
Get everything hot and melted down and use a flathead to spin the valve "stem" to seat the valve.
Also, how clean is your lead? I've had issues with dirty lead causing the valve to stick slightly open.
the key is how to set it right
I have the opposite problem.
My Lee pot finally plugged up solidly and nothing comes out.
Despite my best efforts to dislodge whatever is in there, I was unable to and finally gave up.
I removed the handle bail and valve stem and use it with a Lyman "egg" ladle.
It's remains a reliable melter though.