What could be the problem when I throttle up on my heli and the rotors spin for a second then stop? I have a Hiko 400, all batteries are charged.. Thanks..

When the throttle starts failing this way it is a sign of either a failing contact or batteries. But since you said the batteries are fully charged then we can eliminate the batteries out of the equation.
The throttle contact does not make as much contact as it is supposed to and therefore starts up the rotors and then does not continue giving as much power as it is supposed to.
There could be a power flow breakage or a partial contact somewhere hence the initial startup and the stoppage.
Hope this solution has been helpful?