Weak Ejection caused by:
1. misaligned/dirty extractor
2. weak ammo
3. stiff recoil spring
4. on HKs it could be your finger is touching the slide slowing the action.
1. Clean under extractor claw
2. Shoot some +p ammo and see if it does the same thing
3. If the gun is new it may take a bit to break-in the recoil spring, see no. 2
4. Check your grip or give to someone else to shoot and see if the same thing happens.
5. If above doesn't solve your problems call HK and let them figure it out