OP, I'm guessing that you have to hammer at the sprue plates to get them to open.
A few things can cause the Lee sprue plates to be tough to open:
-Not lubed (use graphite dry lube, or even color on the bottom of the sprue plate with a pencil)
-melt is too cool
-mold is too cool (I don't start casting until a mold dipped in the melt no longer allows lead to stick to it. If lead sticks to the mold, it's too cool).
The first few casts with my Lees usually indicate that the melt is still too cool, and popping the sprues on them requires more force than normal. Cast slightly hotter, and the problem should go away. Note that this doesn't mean you need to turn up the temps-just wait for the melt to reach full temperture, and don't throw in cold lead constantly.
I'll also add that I smoke all of my molds heavily. It helps, but not as much as casting a bit hotter.