Sigh.Rats are incredibly intelligent. They'll take a few nibbles of a new food and then leave it. If it gives them a stomach ache or other negative symptoms, it's likely they won't touch it again. That's one reason why RatX only works on super hungry rats; they're less likely to test it first, thus they'll get the full dose. Your rats have probably recognized that RatX isn't safe to eat.
I am definitely being outwitted by them!