I HAVE had this happen before. About thirty years back. That's when I found up three things could be at fault, undercharge, cool primers and primer up condition. The last can be solved by tipping the muzzle up before firing and tap the butt to seat the powder back on the primer. If the flash isn't submerged in the powder you can get a hang fire or this cooked powder you have. The cool primer we've discussed and a light charge can also have the exact same effect. Or, the extra space in the case can blow it up...