Start slow

My mac was bought last week, with the latest iOS 10.10.3, 1.6 GHz Intel Core i5, 4 GB 1600 MHz DDR3,and I have found the start bottom is not smart. The notebook sometimes be started in seconds, but sometimes nothing happening when I press it.

I did the apple diognose by myself at home, but it turned out no problems be found.

What's wrong with it? I suppose I should go to apple store and change for a new one right?