There could be multiple reasons behind this, and one needs to understand that the poison might even take up to 7-10 days to kill the rat. Repeatedly using the same poison such as warfarin could develop resistance in these creatures, and it is ideal for switching over to single-feed anticoagulants or others. You have switched over, but if the problem persists, maybe the bait quantity was not good enough to kill the rodent. Empty bait traps are indications that the rat is finishing off its meal, and you can continue placing the baits and wait patiently. The rats might move the bait foods to the hiding and take their own time to finish or even waste it. Hence, choose single-feed pellets that could be finished inside the bait station itself such that the rat is sure to ingest it before coming out. Despite all this, if the bait/trap doesn’t work still, maybe, it’s time to switch over to other products that could be more effective.