A:Javier, from our experience, it takes a little time and patience to get that o ring to seat correctly. It is not easy. We usually go through a small variety of small screw drivers and picks before, finally, finding success. The best way to avoid this problem in the future is to lightly coat the o ring with grease before trying to insert the fuel line fitting. We know this is of no help to you now. But, maybe you can feel better knowing that we feel your pain! Good luck!