Sounds like your barrel O-ring is worn.
Check the little rubber O-ring thats seats the bolt prod.
Replace it with a original from hatsan from
www.aurgunseurope.com or just get a generic 7x1.75.
-10.5
Thats 7mm inner diameter, 1.75mm thickness and a other diameter of 10.5mm. incase of a .30 cal (.7.62)
Buy a little extra they are cheap.
You dont have to remove the barrel to get to the O-rings. Simply take a small flat screwdriver or something and fish it out. To put it a new one simply get one part into the grove, use the hexagon shape of the back of allan key and press and lead the rest into the grove by making s little circle (this needs some practise)
If this doesnt work remove the side plate on the right side (watch out those little screws are easy to strip. Correct size screwdriver and pressure are needed)
Now you will see a little plate that pivots. When you remove the cover it most likely Schoots or falls out a little because the hole in the cover holds the Pivot point in place. Clean it. And oil it and make sure you place it back so that the flat side of the pivoting plate is facing down to the gun and the little silver pushrod to the barrel end of the gun is sandwished between the pivot plate and the bolt. (Theres only one way for it to fit anyway).
Now place the cover back and gently tighten the soft metal screws. Rack the bolt up and down and feel if its smooth. Flip the full auto switch a couple times and shoot it.
Let me know what happens