You may have added more syrup than the cupcake can absorb. It's a good idea to pour in a little at a time, watching to see how the fast the liquid is absorbed. When this starts to slow down, stop. Don't be tempted to use up all of the syrup. If you have a little left over, that's okay. You can store it in the fridge for around a week, or pop on the kettle and use it in a cup of coffee or tea.