At first glance this sounds like nothing to worry about. It is possible that since he is used to live rats coming into the tank and having to strike quickly he was doing that at first out of habit. He may not have realized yet that they aren't alive. I sort of had the opposite happen with mine, he used to do what yours is doing, just lazily nose around until he decided to swallow it. He would hide if I moved the rat for him and would only eat it while it was still. Recently he started striking and constricting the F/T rats. I was curious why this might be so I posted about it on here. Basically what people said was that it was more common for him to not strike or feed aggressively because he knows it is dead and that he doesn't need to kill it. Basically with F/T he may just be learning that he can take his time.

As for missing strikes, my guess be the heat signature. If you post how you have been preparing the rat it may help us give some feedback on that, but it does sound like he doesn't have good enough "vision" of it to be accurate. You can try dipping the head in water that has been microwaved for about a minute to boost the heat signature of the head and better imitate a live rodent. This sometimes helps. Also, if like you said he takes a while to get interested, the prey may just be cooling enough by that point that it is harder for him to recognize it.