I agree. Most poisons work slowly.
Also, rats store their surplus food if it is abundant so a lot of poison may disappear but they probably have a stash. If you see one rat, you are likely to have 6 somewhere near.
Most rats are far to intelligent for ordinary rat traps. Poison is still the most efficient way to dispose of the blighters. Have you read the instruction on the bait pack? Some poisons makes them drink to excess and destroys their kidneys. Are you providing some nice water for them?