Please note that the stars you win during the game are not added to the purple star at the top directly. The purple star shows your current level in the game
To reach new levels you need to collect different numbers of stars. Please tap on the purple star to see how many stars remain until the next level. The stars you win during the game are added to this drop-down star box (as in the screenshot). As soon as you collect the required number of stars, the purple star will fill in and you will level up.