I'm a newbie but I've read a lot about it. Someone with actual knowledge can tell you if this has any truth to any of this. I've read everything you tried about switching over, but also
1) when having trouble switching to F/T use a new freshly defrosted rat each offering, don't try to recycle the same one after a failed attempt
2) making sure the HEAD of the rodent is very warm with a hair dryer or something makes it more attractive to the snake sometimes
3) feed in a quiet, dim environment, but do move the rat a bit to get the bp's attention