The good thing about this kit is that if the crystals are not to your liking, you can start the project over again.
Crystal growth will be random, both large and small sizes. If you are not satisfied with the size or shape of your crystals, place everything back in the container and microwave for 1 minute. Stir to dissolve your crystals and start over.
The water needs to be hot. If only warm, it may not dissolve as much as it should.
Heat and humidity in the room can make a difference, as it could effect the evaporation rate which creates the crystals. If it's very humid, try using a small fan nearby (though not pointed directly at the container).
Loosely cover the container with a paper towel. Make sure the tray is not bumped at all. Even vibration could prevent the crystals from forming. It should remain totally undisturbed. Maybe try setting it in a different spot, that could be the issue if the crystals are not forming.