Boosters and coins can give you an extra edge to finish a level, but these are entirely optional. All levels can be completed without them, and our development team doesn't release new levels until they've been tested that they can be completed without using boosters, or making a purchase.
The candies that drop down in the are random, and some levels are designed to be more challenging than others, so you may complete some levels very quickly, while others might take some more time.
If you're still having trouble, please submit a support ticket in the game, by tapping the "gear" icon on the map screen, and then "support," or you can open an online support ticket here.