First thing I would try is just a straight stick worms no rods. (you can keep remelting, until you get it right)
Use a lower temp. and lighter pressure when shooting. Try shooting without the vise. If you have to use the vise, try using a clamp on the top.
No salt in the core. Make sure you have clean/clear opening at the head of the worm.
350* for the cores should be okay. You might be shooting the cores with to much pressure also.
If you have hot plastic and a open core, they should shoot fine.
Good luck, keep trying.