A very common error made when jumping out of the RTF Quad world into the Custom or Scratch build aspect of this hobby is installing the motors in the wrong place. I did it myself along with most that frequent this fine forum. Another common mistake is having the motors in the correct spots , but failing to move the props too. This problems symptoms are usually a build that will not lift off no matter how much throttle is given since the props are pushing down instead of up.
I had a chance to make a few references tonight and will post them below: