Nick:
Please understand the difficulty in making any accurate diagnosis from afar. Sounds like, from your description, a possible broken main spring, broken spring guide or cracked, broken or disengaged cocking pawl. If the problem is due to a manufacturing flaw, the manufacturer, in this case Gamo, will never know if they never see it. Thus, you may want to explore your warranty option, if you have one. Recognition of "field failure" in any volume spurs most reputable manufacturers to improve quality control which could help our fellow Gamo purchasers in the future.
Please keep us posted.
Mike (Machinist)