In the US self raising flour contains ½ tsp less baking powder per cup than UK self-raising flour. If you are based in the US use plain or all-purpose flour instead and add 2tsp baking powder.
If the caramel sauce is a little stiff after being stored in the fridge place it in the microwave for 20 seconds to loosen it up and make it easier to spoon into the muffin cases.