Vostro Power Button Issue

Bought a new Dell Vostro 3520 i5 version

The computer is completely off. When I normally press the power button, it does not turn on. When I plug in the ac power and then normally press the power button, it turns on.

Again, suppose computer is fully off. Instead of normal press, when I press button a bit hardly, it turns on.

After computer turns on and windows logo appears, when even when I press the power button normally, it goes to sleep.

My question is, when the computer is off and ac is detached, why does not the computer turn on when normally pressing the power button, whey normal press work when the ac power is plugged in?

Is there power issue?