The bolt not staying open means that you don't have a spring which is so weak that the bolt is flying past the ammo before the ammo can be lifted. You simply have a gas situation which could be
A. A leak like the gas lock screw
B. Out of spec piston or cylinder
Now, this said, I know nothing of the ammo you're shooting and it could be lame stuff. Try some HXP and see what happens. If you still have the issue, try a slightly worn oprod spring.
Just tough to diagnose with no gun in your hand :)