There can be several reasons for this. It could be because the old dummy was used for too long and became very large compared to the new one. This is the reason why dummies should be replaced every 4-6 weeks.
It could also be that you have accidentally ordered the wrong size. If a dummy is larger than your child is used to it can take some time to adapt and that can be challenging.

You can find more information about sizes under the selected model on our online shop.