Umarex Gauntlet problem

Guys I am new here, i just purchased this gun and sighted it in i then refilled the tank from a scuba tank and now when i pull the trigger i get a "hiss" of air released and the pellet is stuck in the barrel, i have re-cocked the bolt and fired but i just get a low pressure hiss ....any thoughts (BTW there is 2500 psi in the tank)