When I overhand serve, it go almost all the way, just skidding on the net, what can I do?

I have my form down, and all the techniques, I am thinking that maybe I am just not following through. I am in eight grade and I want to make A (no not a team, but the A team, like there is aso a B team) team this year, but all I have to do is get my overhand serve down, I have 3 weeks to do it. Any advice, on ways I can make it actually go over the net. by the way, only rarely will it go over the net.