Lee made a change on at least some of their bullet molds,

and it's a pain.  The sprue plate is now held on by a single pivot/tension screw

that has no way to lock it.  Which means(at least on the two I have) that the screw loosens pretty quickly, and tightening it just means it loosens again after a couple of casts.

Here's my fix:
Go to Ace Hardware and get a couple of 4-40 setscrews.*
Take out the pivot screw and remove the sprue plate and washer(don't lose the washer) from the mold.
Mark the side of the left block about 1/4" down from the top and so the hole you drill will run into the screw hole.
Drill a suitable-size hole, in this case for a 4-40 screw.
Clean the hole out, then use a tap to thread it.  Don't forget to use cutting oil.
Put the plate, washer and screw back in, and adjust to a suitable tension on the plate.
Install the setscrew, lock it down snugly.  Remember that 'snug' is the key here, the block is aluminum, therefore soft and it'd be easy to strip the threads.
Mostly done.

Next time you cast, make sure that the tension is correct once the mold is up to heat, adjust as needed.  Then I'd suggest using a small centerpunch to stake the setscrew in place.  If you've not done such, that means set the point on the block about 1/16" from the setscrew, angled toward the screw, then give it a couple of taps.  You're actually moving metal over so it is forced against the screw.  Do the same thing on the other side, and done.  Here's how mine looks

Yes, I should've thought to take photos of the process.  At the time I was focused on "Fix the damned thing", and that didn't occur to me.


*I had a 4-40 tap, so used that size; you'll need a small screw, but it can be bigger than this.