I mention handles because they control the force of opening and the direction that force is applied. Mold blocks are designed to separate when pulled directly apart in addition to freely pivoting on the mold to handle pins. It doesn't take much of a bind to cause that separation action to be off and a good reason to use handles designed for the mold, especially if it is a quality mold and the tolerances are close.
Handles may not be the problem, but on a new mold it is definitely the first place I would look, especially if I could separate the mold by hand easily. Why do you think the H&G company always fitted handles to the mold and a good reason not to buy an H&G mold where some sleazebag separated the original handles so he could sell them separately.