selmerfan:
(Are you a brass player? Selmer is a maker of brass musical instruments IIRC)
First, check to see if the inner faces of the handle jaws are hitting the bottom of the block slots AT THE REAR. This can prevent the jaws from fully squeezing the blocks closed at the front, and commonly arises in using the older "small-block" handles on the larger blocks; or sometimes it happens even with the correct handles - several of my handles have had to be "fine-tuned" over the years. The solution is to file a taper on the INSIDE REAR face of the handle jaws, from about 0.020"-0.030" at the back to zero opposite the mounting screw holes. Also, check that the wooden handles are not contacting each other as you try to close the mould. (If the blocks gap at the back, file from the FRONT of the jaws.)
floodgate