I only had to switch 2 of mine and they both took F/T the first time. Something that I noticed, and maybe it's just my boas, but one is real picky about the temperature of the rat. My Dumerils, he doesn't care. As long as it is thawed, he will take it without me heating it up any. The Caulker Cay will constrict and eat with only a 20-30 second blast of the hair dryer on the prey. The Costa Rican, on the other hand, prefers it alot hotter. If it isn't hot enough, he will constrict it but not eat, in which case, I just wait about 2-3 hours and re-heat the rat and try to get it in the enclosure quicker. Usually it takes about 60-90 seconds with the blow dryer to get it warm enough. I can only guess that because their room temp is about 80F, I am basically just heating the rat up to somewhere above room temp. So, if you notice your boa constricting and then not eating, that's something to consider...