I’ve completed all levels up to and including level 17 mastering all scenes etc. no more stars to..

You need to level up more by increasing the flower value on your island. Each item you buy for your island has a flower value indicated. How many more flowers you need to reach the next level is indicated in the top left corner next to the flower icon. If you buy a decoration, you need to wait for the construction to finish and to remove the red ribbon before its flower value will count. Other ways to get more flowers include renovating landmarks (i.e. lighthouse), upgrading 3x3 and 4x4 buildings, sending Jack on certain deliveries, earning rewards from timed events and Memoirs Album.