you might want to be a bit more specific..... Perhaps your shoulders are a little unflexible which means that you need to do some shoulder flexibility, so get your coach to give you some..also your back might not be very flexible...again ask your coach for some stretches.
Or maybe try walking down a wall using your hands...so you start facing away from the floor, and then walk your hands down the wall just like a backbend
Another drill is to get over the fear of doing backbends (going backwards isn't really what people call 'natural') so do some back bends starting on the floor and going onto a crash-mat with your hands
First, build up your back and shoulder flexibility by doing bridges. Try to push higher and higher and hold them for longer. Once you have a better bridge, try doing backbends on soft serfaces, a mat, a bed, even a couch. Don't freak out, just go for it! As long as you do it somewhere soft, you won't get hurt. If you can do handstand-to bridges, they also really help, because it is like a reversed backbend, so it helps you to have flexibility in the right places.
Hope this helps, and good luck!!!
*work bridges to gain flexibility...try rocking while in them too
*walk hands down the wall & back up
*do back bends onto a soft, higher surface than the floor to begin with
*HELPFUL TIP:: make sure you are holding your arms up by your ears & are stretching tall even while bending backwards...also watch your fingers all the way down to the floor (this is VERY helpful)!!!