When you are using these replacement plier dies in conjunction with the pliers that have "GF" embossed on them, the white rubber head is intended to be used only for size 14 regular snaps. They are too high and do not provide enough clearance for caps with taller prongs than the size 14 regular. You will need to continue to use your shorter, old-style black pressing head for other sizes. However, you can slice a bit off the top of the white rubber head to make it shorter if you would like to use it for other snap sizes.
If you're using long prong caps, then you'll need to go back to using the shorter, old-style cap die in order to have enough clearance.
New dies can be purchased here: