Insuficient or worn bite . Nothing to do with the spring or the gun being on face . It needs to have a new bolt fitted . Essential to check that there is a clearence between the top of the action and the barrel wings when the new bolt is fitted ,because as the gun wears the barrel drops down and bears on the action rather than on the face .
It is a not uncommon fault with Brownings and their Miroku offspring . I also come acros the problem with other bottom bolting O/U's of other makes .