What make is the mould? Reason being, Lyman use slotted screws while others, like RCBS use Allen head screws.
Let the oil work. As the mould is already on the handles, there is no access to the screw except the screw slot or hole
in the head on the bottom (or top is a tanner mould).
Sounds as if the screw is rusted into the blocks. Give the penetrating oil a chance & let us know the result.
Using a hammer on the end of the screw driver can also help.
Acetone/ATF mix 50:50 is a very good penetrating solvent, & is better than Kroil, although Kroil is very good.