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The SIG 556 and pretty much any other semi-auto rifle that has a last round bolt hold open is going to close the bolt if you bounce the butt with no magazine inserted. The bolt hold open is under spring pressure as well as the bolt. If the bolt is pulled back with the bolt stop engaged the bolt stop will drop under spring pressure. Depending on the spring weights and weight of the bolt will depend on how easily the bolt will drop.
The reason the bolt does not close with the empty mag inserted is because the mag follower is pushing up on the bolt hold open to keep it engaged with the bolt. If you do this with a loaded magazine it will strip off a round and chamber it making the gun ready to fire.
The amount of force to get the bolt hold open to drop will vary and is dependent on spring pressures and the lubricity of the two surfaces.
You should check your bolt release and see if it operates to its full travel by the mag follower. If it doesn't try a different mag or lubricate it to see if that helps the travel or manually move it up and down. Just flip open the receiver and insert the mag and observe the travel. Maybe a weak mag spring as well.
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