The bumper also helps keep the mainspring from bending sideways.
Usually pistols of this type have to be cocked to pierce the co2 or it will gas out because of the hammer holding the cupseal open. If the pistol can be recocked and shot again, then it's likely that the spring from the piercepin block to the nose of the cupseal is way too strong.
The stuff in this thread about tuning the springs of a zeus apply to the delta68 as well: zeus type spring tuning
Hope that helps too!