Dg is right, something is not correct on your shocks.

If you remove the shocks your arms should move up and down freely, if you raise one up and it stays there and does not fall you need to loosen them up a bit.
Keep in mind as you work that there should not be any slop in the arms either, they should move freely though.

Before you install your shocks be sure they operate properly, if you push a piston in it should rebound slightly but should not shoot out to full extension.
You should also be sure that you dont have any air trapped inside. If you operate the shock you will hear bubbles or little squeeks as you move it.
I always let my shocks sit for a bit with the caps off when I fill them, a few minutes and the bubbles usually go away.

keep in mind that the job of the shock itself is to manage the movement per the shock oil, not cause rebound. Thats the springs job.
With shocks installed you should push down and get some rebound depending on the amount of travel you have.

Dbow