Have you tried to heat the f/t up with a hair drier (after defrosting under dry conditions)? My gal will *not* take a f/t if it is wet. It has to be dry and fluffy!!
This ^ is a good start. Your snakes may prefer the PK due to the body temp. Try heating your FT in the same room as the snake, using a hair dryer. Get the body warm, and aim to get the head around 100F (slightly warm to your fingers).